In rP23514/D2492, there was the entity component Autobuildable committed where all prototype methods except the message value handlers were specified using the class syntax keyword.
See comments on http://irclogs.wildfiregames.com/2020-02/2020-02-26-QuakeNet-%230ad-dev.log
The specs https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes don't seem to mention it,
but https://javascript.info/class does ("Class methods are non-enumerable"), and
an Object.defineProperty(AutoBuildable.prototype, 'OnValueModification', {enumerable: true}); does confirm
that the prototype methods specified using class syntax are not enumerable (contrary to the prototype syntax).
Since Script_RegisterComponentType_Common iterates only through enumerable properties, it won't find these handlers and thus not subscribe them.
This can be fixed by iterating through the known handler names and probing if they exist.