Sounds like a security issue. Especially since you mention that a microflow retrieve gives a different result from simply using the XPath constraint on your dropdown reference selector.
The times that I encountered these scenario's, it all came down to the User Management setting for the User Role as specified under Project Security. See also documentation.
In short: the user management part of your security also determines what shows up in reference selectors. If you grant UserRole X no UserRoles that it can 'manage', they will also not appear in a reference selector dropdown.
I just tried this and it seems to be working for me. I made a test project that you can download here. See the added reference selector on the Account_Edit page.
My initial feeling while reading your post was security, so I'd recommend turning it down to Prototype mode and giving that a shot. If that doesn't work, review my test project and see if you have any differences.
Thank you very much - your example indeed works and I will check my project again.