I just finished the Fall of the Outcasts monolith timeline and I have encountered a bug for the first time (and I have fought this boss dozens of time with my other characters before) with my Beastmaster. And after the Timeline boss spawns and the fight starts, I was unable to attack the 4 soul vessels in the corner. When attempting to hit the vessels with Tempest Strike, my weapons couldn’t hit (I have tried with an axe, a club and a hammer). I thought it was a “melee attack area” issue, so I have increased it with crafting, but any weapon I was trying was just hitting the air. Until the boss gets closer and whenever my weapon hit him AND the vessel at the same time, then I was able to do damage to the vessel (even with the boss being invulnerable at that time).
edit: I am playing on controller, just in case it makes a difference in your troubleshoot. I didn’t try on MKB to see if it was also an issue