As you can see, the event names suggested in /script run __on_ have names of the form __on_carpet.script.CarpetEventServer$Event$<event number>@<event hash>, which is different compared to 1.4.12 which has proper human-readable event names, which means they don't work with /script run.

Example image:

Modlist (MC 1.16.4):

blimey, how didn't I notice that? That is a carpet-only issue.

Can also confirm for latest commit.
May have been introduced by 5564363

You can still use them by their human readable name, they just won't be suggested, but they work.

oups