In a lot of communities there is frequently one jackass in power while everyone else in power is cool, and being able to ignore that one person is helpful to avoid their behavior from spoiling the fun of the community for me.
If the "cool admins" allow one asshole to be mod, maybe they aren't as cool as they seem to be or they don't care enough about their own instance, either way jumping ship is better than dealing with that shit.
Blocking means you no longer see their posts, and they can't comment on any of your posts. If they've already commented or replied to you, the messages are deleted (hidden?).
It sounds like Sync is either blocking users client-side (which would be confusing, since server-side blocks do exist), or it is trying and failing to add a block server-side but suppressing the server's error message.
either way, it sounds like a bug.
do you know where the project's github is so someone can open an issue about it?
Just use an account from a different server and then block the admins before you post. That way they can't give you any problems or censor you.
It's reason #4647386 why Lemmy and federation itself is broken.
But you can and really should do it. Since everyone refuses to fix or even acknowledge the serious problems with the platform, it makes sense to simply exploit them for your benefit.