Playing a world with a friend, we recently completed the available bosses and reopened the world today to check out the new update content and give Boss Rush a try. We were progressing just fine, but when the Destroyer was spawned, it automatically de-spawned instantly over and over, thus not allowing the Boss Rush to continue. After giving up on fixing it and stopping the Boss Rush, I looked in this discord for anyone reporting a similar issue and came across this person (#calamity-mod-talk message) saying it did not work for them in multiplayer. Sure enough, it does not work when summoned standalone outside of Boss Rush either. The Destroyer exists for roughly a second or two, and then promptly de-spawns.
I can only assume this is a new bug, as we completed the Destroyer fight prior to this just fine.