It was possible to consume Power Cubes with USE even when the XM Tank was full.
This happened right after Trek and treat Halloween event started so assumed that this was a Halloween Trick.
I was able to "recycle" 20-30 smaller Power Cubes this way.
But after App restart it was not possible to consume Power Cubes any more with USE while XM Tank Full. It showed "Tank Full" correctly:
Samsung S8 Android 9 2.58.1