I can confirm having the same issue since upgrading to ArcGIS Desktop 10.6.1 from 10.6
It affects any new FGDB and Relationshipclass created in ArcCat 10.6.1
Name: rel_streetnames_streetsegments
Origin object class: CAD_GEO_ROW_DBSTN
Destination object class: StreetSegments20190214
Type: Simple
Forward Path Label: StreetSegments
Backward Path Label: StreetNames
Message propagation: None (no messages propagated)
Cardinality: 1 - M
Has attributes: No
Origin Primary Key: STREETID
Origin Foreign Key: STREETID
Selecting all street names in CAD_GEO_ROW_DBSTN, and pushing the selection through the relationshipclass, selects all StreetSegments20190214 correctly.
Selecting all StreetSegments20190214, and pushing the selection through the relationshipclass, does NOT select the correct street names in CAD_GEO_ROW_DBSTN. Only 22 of 3227 street names are selected. About 3210 should be selected. This has been going on every time I've created the relationshipclass between the FGDB featureclass and standalone table in a new FGDB since the 10.6.1 upgrade