(CNN) Boeing allegedly withheld information about the potential hazards of new flight-control features on the model of plane involved in the deadly Lion Air crash in Indonesia last month, according to a US pilots group.

The Allied Pilots Association rejected Boeing's assertion that a safety bulletin issued last week was meant to reinforce procedures already in the 737 MAX flight manual.

"They (Boeing) didn't provide us all the info we rely on when we fly an aircraft," said Captain Dennis Tajer, a spokesman for the group. "The bulletin is not re-affirming, it's enlightening and adding new info."

The manual did not tell pilots that, when the plane's computer detects the aircraft is in a stall, it automatically triggers a response, such as lowering the airpane's nose, to prevent or exit the stall.

That is critical information for pilots, which the association says was never communicated to them.

The Allied Pilots Association says while there are no immediate safety concerns about the Boeing 737 MAX "the fact that this hasn't been told to pilots before calls into question what other info should we know about this aircraft."

The news was first reported by the Wall Street Journal (WSJ).

The Federal Aviation Administration (FAA) said it is not conducting a probe separate to the ongoing Lion Air accident investigation, of which it is a part.

Citing "safety experts involved in the investigation, as well as midlevel FAA officials and airline pilots," the WSJ reported Monday that the automated stall-prevention system on Boeing 737 MAX 8 and MAX 9 models -- intended to help cockpit crews avoid mistakenly raising a plane's nose dangerously high -- "under unusual conditions can push it down unexpectedly and so strongly that flight crews can't pull it back up."

According to the WSJ, Boeing told airlines that such a situation "can result in a steep dive or crash -- even if pilots are manually flying the jetliner and don't expect flight-control computers to kick in."

Captain Daniel Putut Kuncoro Adi, Managing Director of Lion Air Group, told CNN Wednesday that the airline is "collecting all information in our internal investigation, we were working with our experts in (operations and engineering) departments, but again we do respect the investigation process done by (Indonesian safety inspectors), and we look forward to the report."

Issue with AOA sensors

Boeing released an operational bulletin last week, warning all airlines about how to address any erroneous readings related to the angle of attack (AOA) sensors. The FAA later issued its own directive that advised pilots about how to respond to similar problems.

An AOA sensor is an instrument, similar to a small wind vane, that sits outside the plane just below the cockpit and sends information to its computers about the angle of the plane's nose relative to the oncoming air. The sensor helps to determine whether the plane is about to stall and dive.

Boeing said the operational bulletin pointed crews to "existing flight crew procedures to address circumstances where there is erroneous input" from the sensors involved in the Indonesia incident, the AOA sensors.

In a statement following the WSJ report, Boeing said: "We are taking every measure to fully understand all aspects of this incident, working closely with the investigating team and all regulatory authorities involved."

"We are confident in the safety of the 737 MAX. Safety remains our top priority and is a core value for everyone at Boeing," the company said.

"What seems to have happened here is that a new version or a modified anti-stall capacity was added which pushes the nose down automatically. If it's true, it is beyond comprehension that Boeing did not tell the airline and pilots about this," said CNN aviation correspondent Richard Quest.

He added that if important information hadn't been communicated to pilots, it would be a matter for aviation regulators, rather than individual airlines.

"The issue is how much information to give the pilots about the systems on board so they can respond in an emergency," Quest said, adding that pilots are often overloaded with readouts and signals from multiple devices and monitors that can risk distracting them at the worst possible moment.

Lion Air sensor replaced

Last week, investigators working on the Lion Air Flight 610 crash -- in which 189 people died -- said the AOA sensor on board the aircraft had been replaced the day before the incident , but problems persisted.

Indonesian authorities confirmed last Wednesday that the AOA sensor was replaced after a flight from Manado, in North Sulawesi, to Denpasar, Bali, on October 28. The Boeing 737 MAX 8 then made another flight to Jakarta that same day, and the pilots reported further problems.

Investigators said the jet experienced problems on its last four flights -- including, crucially, the flight that crashed, according to Soerjanto Tjahjono, the head of the National Transportation Safety Committee (KNKT).

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Search for voice recorder

More than three weeks after the crash, authorities are still searching for the plane's cockpit voice recorder (CVR), which is believed to be buried under deep mud. If found, it should reveal what happened in the cockpit in the final seconds of the flight.

Investigators are already examining the flight data recorder (FDR) that was pulled off the sea bed , some 30 meters under water, on November 1. Preliminary findings suggested there were problems with the air speed indicator on the past three flights before the crash.

The airspeed indicator is like a speedometer and tells the pilot how fast the plane is moving through the air. It does this by gathering data from the plane's pitot tube and the static port, which essentially compare differentials in the air pressure to arrive at the plane's speed and the altitude.

After problems were reported with the air speed indicator, the AOA sensor was replaced by a Lion Air technician in Bali before the plane departed for Jakarta on its penultimate flight.

Passengers on that flight told CNN that the plane experienced a significant drop in altitude shortly after takeoff. "After 10 minutes in the air the plane dropped as if it was losing power. People panicked. It dropped about 400 feet," passenger Robbi Gaharu said

Once in Jakarta, a Lion Air technician checked the plane again and gave it the green light to fly on its final flight, from Jakarta to Pangkal Pinang.