Ya, I agree it's acting like multi-part, but it's not multipart. Unique parcels/shapes. If I "Keep All Target Features" it works in ArcGIS Pro (other than being painfully slow, almost a full minute to open the table). So I guess I could do that and then just use a definition query to get rid of seeing all the ones that didn't join.
FYI, the exact same process, data, join, etc...works perfectly in ArcMap 10.5.1 and is not slow at all. I can "keep common" and select/pan/zoom through the table with no problem.
Thanks for all the help suggestions!