This is 100% repro-able (for me at least)
Using krel, attack the boss just before the cinematic begins.
For example if I fire off a supernova towards the location that the boss will spawn in, just before the cinematic starts. It will hit the boss right when the cinematic begins and then the boss will never become visible, and will even start attacking me during the cinematic.
An even easier way to repro this is to make a squad with krel and blitz
Using krel, make sure plasma wreath is cast, and then run toward to where the boss spawns in. The wreath will hit the boss just before and during the cinematic, causing the same issue.
I was able to cause merak, polaris, and even the corrupter to become invisible like this.
when the corrupter resurrects, he becomes visible again
The visibility problem is for all players in the party, not just myself. Its similar to a desync problem, where you can see where the enemy is attacking from, but you cant target them. So we had to rely on projectile attacks which could be manually aimed in order to damage the boss.
Needless to say that it made for some amusing boss fights