Answer
This is a genuine, confirmed gap rather than a configuration mistake — directly starting a workflow from a custom button via a custom event isn't supported the way the equivalent PL/SQL-triggering pattern was on Apps10. This has been raised as a feature request ('Execute external integrations via command button') and was expected to be addressed around the 23R1 timeframe — check your release notes to confirm whether this has since landed if you're on 23R1 or later, since it wasn't available as of the release this gap was reported against.