Instagram is investigating an issue that’s causing some of its most popular accounts to appear to lose millions of followers.

“We’re aware of an issue that is causing a change in account follower numbers for some people right now,” Instagram wrote on Twitter this morning. “We’re working to resolve this as quickly as possible.”

Popular YouTube creators, including James Charles and Zach Clayton, who each have millions of Instagram followers, began tweeting about the issue last night. Other popular Instagram users, including celebrities like Ariana Grande and Selena Gomez, saw losses in the millions.

“Why did I just lose over half a million followers, Instagram,” Charles tweeted. “[What you doing], sis?”

A report from Fast Company originally suggested that “Instagram took actions to remove millions of accounts the company believed to be inauthentic,” but it’s unclear if that’s the case following Instagram’s tweet about the issue. Many companies, including Instagram, do sporadic sweeps of their user bases to remove spam or inactive accounts. The company announced in November that it was gearing up to reduce the number of inauthentic accounts on the platform.

“We will begin removing inauthentic likes, follows and comments from accounts that use third-party apps to boost their popularity,” a blog post from November reads. “We’ve built machine learning tools to help identify accounts that use these services and remove the inauthentic activity.”