I tested it in 2.5.8.1
In such cases, I get a warning like this
property xpath constraint on the grid of the pop-up form has no effect when the form is used for selecting. Instead, the selection constraints properties of the reference set selector can be used to constrain the selectable objects (by the way: was there a way to copy/paste the text of this warning? I typed it...)
I think this is acceptable...?