Hi Scott!
Thanks for these questions…
When the problem began I was:
1. Pasting into the attribute table
2. The left side,..I know you can’t edit the table that is used as the join table (the right side)
3. First occurrence was about a month or so ago.
More info,..I was able to export the parcel layer to a shapefile and copy and paste easily in ArcMap 10.5.1. I then brought the parcel shapefile back into a new gdb, and was successful in copying and pasting using the attribute pane. That seemed to work fine. And, so you know, the source data comes from an excel spreadsheet (xlxs version), imported to a gdb.
The updated parcel layer is available to view at https://slvgis.maps.arcgis.com/apps/webappviewer/index.html?id=603b8e4159bb4e13a55960bde76d4f51
And yes, I have reached out to ESRi support, I have 2 case numbers as this is the second time I have had this happen. Thanks for contacting me!
Cheers,
Pete Magee