I don't know about your experience with mirror stance but mine differs drastically to what you describe. Mirror stance, for me, has always given a stealable power on first "contact", assuming I could generate the threat needed to be targeted in the first place.
I did however retry for quite some time, just in case your suggestion of a "chance to learn" might be the solution. I even reran the dungeon with Varena on the slight off chance that the Calise route is to blame. But no, no matter the setup every other power except "flesh to stone" was stolen when hit by it.
Can I assume you were able to obtain the power through mirror stance on your end? Is this only a problem on my end?
The only thing I could think of that might explain the failure of stealing the power is that the initial hit does not deal damage. I'm just guessing at this point