You are not doing anything wrong. I think that this might be a problem with the Lemmy-UI and its compatibility with safari. I don't have any iPhone to test it myself.
Looking through the issues, I believe I may have your issue: https://github.com/LemmyNet/lemmy-ui/issues/1220
To make sure that it is an issue with Lemmy and not with Mander, can you do me the favor of checking if you see the same in a test instance? It is here: https://enterprise.lemmy.ml/
Sorry, hopefully this will get fixed by the next release.