For anyone watching: we dove into this further and it was solved by recreating the Response Mappings on the Zoom webhook (recreated exactly as-was, without recreating the entire 'hook definition).
We didn't have a problem with new webhook definitions in Calon's instance, nor with 'hooks in other instances So as far as we could see there wasn't a systemic webhook problem, just a corrupt definition. (Or if there was also a system-wide webhook problem going on as Support implied, it was unrelated.)
The URL-encoding is definitely necessary w/Zoom (despite Zoom's doc to the contrary) because otherwise you can't pass many valid SMTP addresses.
Thank you very much Sanford! This was an interesting case and I'm glad we were able to troubleshoot together and I am back up working correctly now.