A good discussion is found here: https://musicbrainz.org/edit/59163835

Case: it is somehow possible to add multiple ISRCs to a recording. Maybe this is intentional, but in general it would not be needed nor be the factual truth. These IDs are there to identify unique recordings...

I do encounter recordings with multiple ISRCs frequently, so merging does not warn (enough) when this will occur. My proposal is to at least show a big warning when merging recordings that have different ISRCs. One step more would be to disallow a merge/addition.