It doesn't look like the new record source is being saved at all.
1. Mark a bib record for overlay
2. Go to Cataloguing -> Import Record from Z39.50
3. Find a new record and click Overlay
4. Click on Edit Z39.50 Record
5. Select a record source
7. Click Save
7. Go back into Edit Z39.50 Record and you'll see that the record source field has gone back to "Select a Source". If you made any edits to the MARC record those edits have been saved.
I see this in the 3.1.4 web client
It doesn't look like the new record source is being saved at all.
1. Mark a bib record for overlay
2. Go to Cataloguing -> Import Record from Z39.50
3. Find a new record and click Overlay
4. Click on Edit Z39.50 Record
5. Select a record source
7. Click Save
7. Go back into Edit Z39.50 Record and you'll see that the record source field has gone back to "Select a Source". If you made any edits to the MARC record those edits have been saved.