Divine Font can transform 21/20 gems

Divine Font "Transform a non-Transfigured Skill Gem to be a random Transfigured version" can transform a 21/20 corrupted skill gem into a transfigured version. I don't know what is expected behavior but people been saying it's a bug. Thank you.

Reproduced with 21/20 Elemental Hit gem which was transformed into 21/20 Elemental Hit of the Spectrum. Uber lab, 2 man group.