I am also unable to reproduce it now except for the group mention marketplace app, and our custom tournament page.
What's strange is that earlier I had a 100% repro on any page with the regular mention type. No idea what fixed things, honestly. Especially seeing as it was still an issue after I did the tests with disabled and re-enabled customizations.
Edit: I'll chalk it up to repeated resets of apps and plugins until I find something reproducible in regular areas. Appreciate you looking into this hopefully wild goose chase.
Nakamura RTS's post in Page Record Mentions Trigger Notifications Each Edit was marked as the answer