Possibly related:
I had enough stability in a regular timeline (last ruin) to do both quests and the boss. I finished the first quest and had to log off afterwards. After logging back in I (barely) did not have enough stability to do the boss any more.
This has happened to at least two characters of mine during online play.
Here is a quick video demonstrating the issue:
I just got done with quest 1 in the non-empowered spirits of fire timeline. I ported out, logged out, back in and checked my stability again. It went from 413 to 383.