This issue affects the Head on the Crank and Zap Mech bundle.
Expected Behavior:
As this bundle already has the character’s face included as a Classic Face, it is expected that the head would have support for Classic Faces, as this would allow for the included face to be applied on the bundle’s head and have further customization through other Classic Faces.
Actual Behavior:
This bundle only has Classic Face support in R6, with the face already included in the bundle as a Classic Face forcibly imprinted onto the head when used in R15.
The best fix for this would be to give the head support for Classic Faces in R15. There is functionally no difference for a user wanting to use the full bundle as the face is already included as a Classic Face, and overall evokes better user customization and expression, as people can mix and match the head with faces of their choosing.
It’s worth mentioning that Classic Face support in R6 doesn’t occur with Crank’s other variant, Gearmo and Blast Mech, which has no Classic Face included.
Gearmo and Blast Mech has no included Classic Face → Head has face imprinted in both R6 and R15 
Crank and Zap Mech has an included Classic Face → Head has Classic Face support in R6, yet has the face imprinted in R15 
It’s weirdly inconsistent!!
(also, it being a bug aside, i just think the bundle’s head shape is cute and getting to use classic faces with it would be awesome, which the creator of Crank seems to agree with! 
)