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.)
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