Prime Hologram bugged
The issue is that there is another phase of the fight after this. However, sometimes the event freezes up after you get the hologram to 25% and doesn’t start the next part. It is pretty frustrating when it’s hard enough to even get a chance to fight the hologram.
Living World QA Embed
There is a main forum thread in the Game Bugs Forum about this issue:
https://forum-en.gw2archive.eu/forum/support/bugs/Invulnerable-Prime-Hologram/first