Adding multiple related records to a feature in ArcGIS Pro is a pain in the butt. I would create a record and start entering values, and the same values would be applied to other related records as well. I finally was able to stop this behavior by adding a new record, clicking a different related record, then clicking the record to edit before entering attributes. I don't think I should have to do that. (BTW, this is all being done in the attribute window.) Any tips or tricks to make life easier?
[UPDATE:] I figured out what's going on. When I add a new record, it's not automatically selected: the global attribute form is still showing, and I wasn't noticing that. So I can at least save one mouse click by selecting the new record instead of selecting a different record first.
Thanks for posting! I was having the same issue and thought I was crazy.