i follow several pixelfed accounts from here, mastodon and while most of them are quite new and/or sparsely filled, they are not empty. so, seems to work.
what is yours? i'd try and follow, maybe it's different, depending on instance/server or whatever?

I haven't really found accounts to follow yet, so I can't test much. My account is @LilithElina@pixelfed.de.

i follow now and see one single post, 'hanover running'
is that correct?

That's the one I found by searching for the link directly in Mastodon. The two posts from yesterday seem to have been lost for Mastodon.

Post is lost for Mastodon? I don't understand. @b_age

This one for example: https://pixelfed.de/p/LilithElina/786639018375052745 I can't even find it using the search bar in Mastodon.

No problem over here in Mastodon or Akkoma instance.
@b_age

Mastodon only collects upcoming posts after the subscription, not old ones.