Oh, I found it, it was on the discord. I was misremembering.
Here:
"Okay so I can confirm this fixed the ultimate reverting back bug. Go into your skill tree and completely refund all the skills leading back to your specialization ultimate. Refund the ultimate, and refund your specialization. Then put all the points back into them, go back into your character screen and change your ultimate to your preferred ultimate. It has not switched back on me once since I did this. I have been playing for over 8 hours and it hasn't reverted once."
If this fixes it, let me know so I can add it to their list on discord, supposedly it has fixed it for 4-5 people so far.
