> 1. What is the point to reset `font-lock-major-mode'? Isn't this
>    `font-lock-set-defaults'?
> 2. Why do we need to tell font-lock to update its internals? No other
>    style files do that.

I think the issue is that `font-lock' doesn't recognize the syntax
change for : and _ while the style is loaded.  I admit I'm not familiar
enough with the internals of `font-lock' to tell, but from trying, I had
to look at the internals of `font-lock-debug-fontify' and found out that
setting `font-lock-major-mode' to nil was needed.

Do you get other results if you apply the patch and comment out that
form?

Best, Arash