Adaptive Damage "default" isn't working

Runes like Press the Attack, Electrocute and Arcane Comet deal "Adaptive Damage", which means they deal physical or magic damage based on whether your bonus AD or bonus AP is higher. Apparently, the default is based on the champion you play.

In case of a tie, each champion has a default they will go to based on what that champion usually prefers. Reference

Reproduction has found that the default is always physical. This is regardless of whether the champion's profile states they are Physical, Magic or Mixed champions.

I encountered this bug while testing what the "default" of Mixed champions were - and they were all physical, including champions like Corki whom I expected to be magic. I then thought, "wait a minute" and tested some mages. It always defaults to physical.

Reproduction steps:

1. Select any champion that doesn't have bonus AD or AP on spawn.
2. Select Press the Attack in conjunction with other runes that don't grant bonus AD or AP on spawn.
3. Trigger Press the Attack.
4. Observe that the bonus damage is always physical with no bonuses.

Press the Attack is the simplest way to test because the Domination and Sorcery keystones come bundled with the bonus AD and AP, and thus contaminate the Adaptive Damage results. I'm now patching the PBE to test the new Grasp of the Undying. You could also try to have equal amounts of bonus AD and bonus AP - but this was difficult to reliably obtain, whereas 0 and 0 was easy.

EDIT: The new Grasp of the Undying on the PBE also produces physical damage by default.