I’m not being funny, as had been previously mentioned. A heating eng or electrician (who actually knows central heating wiring....) should be able to give you a definitive answer in less than 5 minutes.... I know you know and trust your electrician dearly, but if they do not understand central heating wiring, or have much experience working on, then how do they know what to test for? That doesn’t have any bearing on them as an electrician, this is control wiring, electricians I know, trust dearly and work with on projects week in week out, don’t like to touch control wiring...!
Something is either not wired right, or is not functioning correctly, both of which will be very evident when checked...
Stu