Amethyst uses 2 controversial methods to censor „spam“. The first is a rough match against a user’s behavior. The second is to look at the users that you follow. How often do they mute a specific user and how often do they follow him. Based on that that user’s posts will be hidden from you. Both methods work independently. Triggering one is enough.

I don’t use that app, but their users claim there are options that they have to switch these off.