Tokyo - Nintendo shares declined as much as 6.5% on Monday after the hit game Pokemon Go dropped from the top of app download charts in Japan.

The mobile game fell to fourth place among highest-grossing apps for Apple devices in Japan, according to researcher App Annie. The shares, which last week posted their biggest decline in more than two decades, touched a low of 20,100 yen before recovering to 21,170 yen as of 1:22 p.m. in Tokyo, down 1.6 percent for the day.

“That’s a surprisingly fast and big drop” in the rankings, said Serkan Toto, founder of consultant Kantan Games.

“In Japan’s app market in particular, there is a big gap between the top 1 and 2, and the rest.”

The share reaction underscores the difficulty analysts and investors have had in gauging the impact of the game on Nintendo’s bottom line. While Pokemon Go has been a social phenomenon, with parks and streets in cities from New York to Tokyo filled with people chasing virtual pocket monsters on their phones, the Kyoto-based company has said that it doesn’t expect a huge boost from the app, which it developed with Pokemon Co. and Niantic Inc. An accessory called Pokemon Go Plus, was also delayed last week.

Japan was the second-most lucrative market for smartphone apps in the second quarter, after the US, according to App Annie. Pokemon Go is still the highest grossing app in the U.S. on both Apple and Android devices, and the top app for Android devices in Japan, according to the researcher.

Pokemon Go was replaced by Mixi's Monster Strike as the No. 1 grossing title, followed by GungHo Online Entertainment’s Puzzle & Dragons and Aniplex Inc.’s Fate/Grand Order.

An update to the app that rolled out over the weekend wiped out in-game progress of some users, the Telegraph reported. The game’s official Twitter account acknowledged that some users were experiencing login issues, but reassured them that game progress remained intact and posted a link to a potential fix to the problem.