I don’t know if that is a bug, so I post in General, but it was weird…
I had just reached enough stability to challenge the boss. I did the first quest echo, then decided to go to the observatory to see if I had any prophecies related to this boss.
When I came back to the monolith, my stability wasn’t high enough for the boss anymore.
Curious…
That is curious. As you probably remember there used to be a loss of stability when you fought the boss and lost but that was changed. There was a recent bug that when you completed 1 or 2 of the quest echoes but didn’t finish by killing the boss, then left and came back you had to start the quest echoes all over again. That was fixed very recently.
I haven’t noticed the loss of stability by leaving though. Are you playing online or off?
It was online.
Oh well, not a big deal, having to do 1 more echo is not exactly life-changing.
Not filing in a bug report as I moved on and don’t have much details to share.
And yes, later on a boss killed me and I was surprised to see my stability unchanged. 
After further testing: the problem is not about leaving, it is about buying prophecies.
Each time I buy a prophecy after a quest echo, I lose around 30 stability. Tested 3 times, on 2 different characters. If I go to the observatory (or anywhere else) but don’t buy anything, my stability remains stable (ah ah).
Bug report section, here I come!
Wow, what a great find. You rock!