The current working theory is that somebody who is being targeted is partially desynced at some point. This somehow means that one of the orbs cannot get a person to a target. I have seen this happen even on Boss 3.
On Boss 3, somebody desynced and they happened to also have an orb over their head. The orb cleared because a sky sun spawned right over them and they didn’t have to move. For the rest of the fight though, the suns went off screen. At roughly 02:04 the person who get the bomb over their head desyncs. You see the bomb clear because he’s close enough. At 03:10 the sun orbs start going off screen to the left. For the remainder of the fight, no sun targets the group. At 03:52 you also see it go off screen.
When it has happened for Boss 4, it seems to stick around way more often than for Boss 3. Once it starts going off screen, it continues going off screen and haven’t found a solid workaround quite yet.