[class.copy.assign] p13 only specifies the effect of the implicitly-defined copy assignment operator for a union class.

The implicitly-defined copy assignment operator for a union X copies the object representation ([basic.types]) of X.

we lack to specify the relevant expound about the effect of the implicitly-defined move assignment operator for a union class. Presumably, that effect is equivalent to that of the above? which maybe that

The implicitly-defined copy /move assignment operator for a union X copies the object representation ([basic.types]) of X.