Seems to be a bug where if you "Collect Reward" on a boss that has been killed, then click "View more bosses" that it may not show other defeated bosses. For instance, right now I'm currently in 49~ boss battles. I clicked on the Bosses Tab and started collecting rewards, then clicked View More Bosses, and collected until I was done, then had to click the Bosses Tab again and it showed me the rest of my defeated bosses. Of course, it could be that players are simply killing other bosses while I'm collecting. Can't rule out that possibility, I guess.
your right...its done that since the onset of the new boss reward collecting page. Its slightly annoying but i was so happy with the update i let it slide...lol
Your will have to refresh before going to the second page to re-populate it. Cause it refreshes when you go to the other page thus if you removed some from the first page then they will shuffle up and not show on the next page.