The one I noticed it the most with is hexbear. I know they have a bad rep but I like to keep an open mind and tried a bunch of times to chime in whenever I saw one of their posts pop up on the all feed i felt I had something relevant to add to.
I also know they are a bit echo chamber-y so I assumed they might have a special hidden restriction in place for non-approved instances. I would be surprised if they actually banned my personal account because I don’t think I wrote anything they might consider controversial.
But the fact remains that (as far as I understand) my feed would not show any of their content if my instance was not federated with them, no?