Since no one has made a post about this (I believe) I'm going to report it myself!
I'm sure that I have participated in a couple bosses before the current unlocked boss that is available at the Guild I'm in right now, but the rewards are nowhere to be seen despite having participated on them. I was suggested through the discord that it might be tied to a boss reactivation issue, but yeah, the rewards from the previous two bosses I am SURE I participated in are... simply not there... for context it was the only time I participated in them - which means, not the reactivated bosses, but the actual bosses. I haven't really tested if fighting the reactivated ones will grant me the rewards, but I will update this if it happens that I cannot claim the rewards regardless of already having defeated the boss once.