I see this often - where I have a response to a comment that hasn’t loaded. Do I have the parent commenter blocked? Or is this a bug on the platform or possibly the app?
As a dev, Lemmy comments are really difficult to work with and sometimes they go missing. Every app tries to mitigate it their own way, but ultimately we are waiting for Lemmy to improve and have clearer documentation on comments. So to answer your question, it’s probably both a Lemmy and Memmy issue.
Also, if you have bot accounts disabled/hidden in your settings, it also hides bot comments