I discussed it on mastodon with someone a bit more experienced with kbin than I am, and apparently it's a known problem that mastodon and kbin/lemmy do not play well together yet, but people hope that it'll get better because kbin and lemmy are still young compared to mastodon so of course they have less working features.

They fact that different instances of kbin and/or lemmy are also having a hard time synchronizing posts is still puzzling though. I know sometimes I need to comment on a post I make to have it appear on kbin, but I don't know why these posts and why it works...