There are a few of us that have been posting inconsistencies with case on globalid.
See here https://community.esri.com/t5/arcgis-survey123-questions/globalid-in-lowercase-from-collector-to-survey123/td-p/822843
and https://community.esri.com/t5/arcgis-pro-questions/globalid-inconsistent-display/m-p/291405
I think I may have traced it down to Field Maps.
If I add a feature in a web map in the browser I get uppercase GlobalID. If I use the exact same map and add in Field Maps it does lower case.
AGOL Add

Field Maps add

Then ArcPro changes the lower to upper for display

So the exact same record shows lower in AGOL and upper in Pro.
The big issue here is my relationship class will NOT match lower and upper so all my relationships are breaking.
At this point I am not even sure the standard? More info in the posts.
Please help I am going nuts.