Vlads argument on twitter for this not being a bug seems to be that selecting entities as part of a projection/multiselect isn't specified.

I just checked the specification. I agree that the examples use simple properties only and also they don't explicitly state that additional selection items must not change the selected rows.
They also describe arguments to a Selection identical to the parameters of a multiselect, and don't put any constraints on them.
And changing the number of results returned just because on selects more or fewer attributes seems odd to me.

Section 6.5.11 of the JPA Specification seems relevant.