Major KOLs like Sơn Tùng MT-P and many other celebrities are facing this issue. Even Facebook’s own Mark Zuckerberg now shows just over 9,000 followers, down from hundreds of millions before.

There are three main theories about this. First, it could be a technical glitch on Facebook’s side causing follower counts to drop temporarily, which they will likely fix soon. Although the follower number on profiles is down, clicking into account details still shows the full count. Another idea is that Facebook is cracking down on accounts that use tricks to boost fake followers.
Some users pay for fake followers, so this could be a sweep targeting those accounts. However, this theory seems less likely since even accounts like Sơn Tùng MT-P and Mark Zuckerberg are affected.
A third possibility involves a draft government decree updating regulations on internet services and social media accounts. The draft would require social media accounts with over 10,000 followers to notify authorities, specifically the Ministry of Information and Communications. But since this draft hasn’t been officially approved or enacted yet, this explanation is probably not the cause.
Most likely, this is just a temporary global technical issue on Facebook’s platform. Their engineering team will surely work quickly to fix it.