I guess it's the second or third post I created here with this issue. Is this only happening to me?

I haven't noticed this behavior. Do you have steps to reproduce it?

This also happens for me with most communities but not C/boostforlemmy apparently

I've found that pinned posts that are ones will always be on top. There was a bug where some posts unpinning did not federate properly, as I recall. It may not be the fault of boost

Open a community, sort by "New"...

It does that every time? Because I tried that before I commented and it worked as expected.

Yes. It's been doing it for months, even years. I never see it work as intended.

Regards to the person that downvoted my response. I would love to know why you did that

Are you filtering out previously read posts? Maybe you've already seen all the more recent posts...

Found out this was happening due to me having the "show read" setting turned off in my account settings. Turns out read≠hidden... That kinda make the clear read function useless if I can't easily decide when I want to see things I've already seen before...