In the align-self and justify-self properties, the auto keyword looks up to the parent's align-items or justify-items property. That's fine.

On justify-items we also have an auto keyword, but it's not there for the general case: it's just there to handle some special computation in case the parent’s value contains a legacy keyword alongside an alignment keyword.

It would be equally plausible to just use legacy as this magic-computing keyword, and leave auto out of the property entirely. Note that align-items doesn't have an auto keyword, because it doesn't have legacy.

Proposal A is to rename auto to legacy.
Proposal B is to remove legacy entirely and leave the handling of <CENTER> and friends to UA-specific magic and/or a separate (Yet Another) alignment property.