Take a look at the config from docker (https://github.com/LemmyNet/lemmy/blob/main/docker/nginx.conf). It proxies to either the frontend or the server based on the request headers/method/path etc.

Is that Memmy? I've noticed similar issues in the feed list -- the same post content shows up under different headings.

My guess is it's a cell reuse bug in the app (not in Lemmy itself).