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?