I'm no expert but I've found that they tend not to pull in old posts when you sub, you'll only see the new ones. And sometimes there's a delay on those coming across too, but they do seem to get there eventually (you see the same thing with people joining in Lemmy convos from Kbin accounts, their comments sometimes don't show up on the Lemmy side for a good while).
If you desperately want to reply to a particular existing post you can search for the URL of the post itself like you did for the magazine, and load it in that way.
p.s. just to clarify this seems to be Lemmy / Kbin behaviour in general, as it's the same on my lemmy.world account too