GuestLastBootTime - Session Group Subgroup Expression

I have a session group using GuestLastBootTime as a session filter.  Works fine.
However, I am setting it as a subgroup expression.

It lists out the datetimes out, quantity 1 for each in general.. but selecting any of these subgroups does not yield which machine(s) match at all.

If I apply GuestLastBootTime as subexpression for other session groups, works fine.

Why?