@malwaretech I think its a bug. I noticed first when there was a discrepancy between the post count someone I followed had and the posts I could see. So I joined their instance and there they were.
At least one person in my feed tested it by following their own alt account and confirmed: not all posts seem to get federated to other servers. As yet we can’t see any logic why one post propagates but another doesn’t. Seems to be random though some servers seem worse. Maybe load?