I popped Aryel’s black activate gem, and she disabled my Yosei, the Morning Star in order to use her ability, instead of the Knight with the greatest power I controlled (which was Aryel).
Any chance you were using Dub? That could make Yosei a knight.
This bug has been around since Aryel came out. The knight clause in her ability appears to be irrelevant.
Yes, she disables the strongest creature you control, not the strongest knight as described.