Bind-time variables cannot be referenced next to another bang:
fails because the two bangs are condensed to one which yields an invalid feature id that ICE03 will complain about.
Triple and quadruple banging does not do the trick. AFAICT, !!(wix.FeatureId) is impossible to achieve.
Either escaping should work like C quoted strings to cover this scenario or $(wix. should be allowed as a prefix as well, perhaps with a warning instead of today's error.