An expected field was not found or could not be retrieved properly & Matching records may not appear in table view due to join validation errors.

https://support.esri.com/en/technical-article/000012711

Matching records may not appear in table view due to join validation errors.:

https://gis.stackexchange.com/questions/337809/joining-my-spreadsheet-data-to-my-geodatabase-file-is-not-including-all-matching

I think you'll need to do a query table instead of a join for 1:m (one to many) joins. 

 

https://community.esri.com/t5/geoprocessing-questions/join-error-i-have-never-seen-before/td-p/580461