Since I am not getting anywhere I went to support. They agree 123 and Field Maps is doing lowercase when everyone else is doing upper. But they say it is not a bug but an enhancement.
ENH-000130624
One possible workaround I am testing is to make any ID field a GUID. GUID does not seem to care about upper and lower so the relationships work. It is harder to use a GUID and it forces me to add the {} and it cannot be a diff text key which is a bummer.
Also even when I paste in a upper GUID it changes it to lower in AGOL
But In ArcMap they are all upper and show as upper.
So AGOL, 123, and Field Maps is all lower or shows lower but ArcMap, Pro, SQL, etc it is all upper and shows upper.
For sure a inconsistency that is very confusing. I hope all the teams can just agree on one choice and use it.
Also 123 uuid() does all lower.
Going to try and get it escalated so hope 123 can make the switch.
thanks