As raised by Reshad on IDR mailing list, the best-path leaf needs to be mandatory, or have a default.
This is motivated by RFC 7950, section 7.6.1:
If a leaf has a "default" statement, the leaf's default value is the
value of the "default" statement. Otherwise, if the leaf's type has
a default value and the leaf is not mandatory, then the leaf's
default value is the type's default value. In all other cases, the
leaf does not have a default value.
As raised by Reshad on IDR mailing list, the best-path leaf needs to be mandatory, or have a default. This is motivated by RFC 7950, section 7.6.1: