Summary
Custom value remains by design even after selection list is deleted.
Question
Once a custom value of a selection list in a deployed custom CP is added by the 'Set Selection List' operation in HP Solution CP,
the custom value remains in the Central DB even if the selection list itself is deleted.
So, if the same selection list is added again after the deletion, the default value of the newly added selection list will not be referred
as an actual value of the list but the custom value will be used instead.
Answer
This behavior is expected and aligned with 9.x.
A workaround would be
1．After un-deploying the old entity, but before deploying the new one, delete the configuration item via REST (with DELETE).
or
2．Remove the custom value via REST (i.e. PUT with null in the request body).