Facebook FB -2.25% continued its series of metrics mishaps, disclosing in a blog post Friday that it undercounted traffic from some publishers who posted content to its Instant Articles platform.

ComScore, a measurement firm that has partnered with the social media giant, said mobile traffic from iPhone users was being undercounted from Sept. 20 to Nov. 30, while traffic from Android devices and iPads wasn't impacted. Facebook says the error was caused by a recent update of its software.

“We have fixed the issue and are working with comScore to produce updated estimates for the relevant time periods for the small group of partners affected,” wrote Facebook in a blog post. “We have reached out to affected publishers.”

The discrepancy is the fourth Facebook has discovered since September.

The metrics snafus have raised concerns for some marketers, ad buyers and publishers. Facebook says the problems haven't impacted billing. Still, some executives say incorrect statistics can affect how ad buyers allocate budgets, and Facebook has been under pressure to allow more thorough and independent measurement by third-party firms.

Instant Articles is a venue for media companies to publish content directly on Facebook, rather than linking back to their own sites. For most affected publishers, the data error impacted less than 1% of their traffic during that period, according to a person familiar with the matter.

However, several big name publishers felt a much larger impact. ComScore estimated that some publishers’ traffic was undercounted by 10% to 20%, this person said. One site’s unique user number was off by roughly 30% in November, this person said.

Among the sites that felt that large discrepancy were the Washington Post, BuzzFeed, Mic, Entrepreneur, Foreign Policy, Inverse, PopSugar and Variety, the person familiar with the matter said.

BuzzFeed, for its part, took the error in stride. Publisher Dao Nguyen said that Facebook Instant Articles on iPhones drives roughly 15% of the traffic to BuzzFeed.com. However, given that a large part of the company’s audience consumes content on platforms like YouTube or Snapchat, which aren’t tracked by comScore, Ms. Nguyen contended that unique visitor numbers from comScore for individual websites “are less and less relevant to the future of media.”

Write to Mike Shields at mike.shields@wsj.com