I have the Power Stone at rank 3, and I like to equip it to one of the boosted 5*s for some extra oomph.
This time it was Victor Mancha (Victorious)'s turn... but it seems bugged. Despite the banner for the Power Stone popping up sometimes, it will NEVER actually fire a power.
Demonstration:
https://youtube.com/shorts/JJ_a4ZILZfc?feature=share
During her boost week I also noticed the Power Stone is "broken" on X-23 (Weapon Hex). It will sometimes fire but it will only fire her green power (Living Weapon), never her Red or Yellow.