Known issue being worked on: https://github.com/LemmyNet/lemmy/issues/3588
For posts, I have been working on it this week and found that it has been broken since June 23 commit. Posts don't even get removed on the home server with 0.18.2, a fix is already waiting for acceptance: https://github.com/LemmyNet/lemmy/pull/3613
As for comments, with 0.18.2 - it depends where the delete is done. If you are on a remote instance from the community, the comment will be deleted on 2 servers, but no more. The local instance you use will delete, as will the community home, but it seems to me there never was any code to distribute deletes to subscribed servers: https://github.com/LemmyNet/lemmy/issues/3625
It also impacts moderators removing posts and comments. As these internally are similar to deletes. !asklemmy@lemmy.ml is one of the communities that has a lot of moderator removals due to people confusing the purpose of it as a technical support forum, and you can see servers not getting the mod removals frequently.
Not issue with Memmy, but lemmy itself. This is how it works. It needs time to propagate into other instances. And sometimes even the propagation can fail.
EDIT: this is not about Memmy. Misread the community. The second sentence still stands.
Lemmy
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For discussion about the lemmy.ml instance, go to !meta@lemmy.ml.