IFCOMET0717P OD-ADS MEA Comet 4C was only released in 2017.
This happened many moons ago with Corgi Aviation Archive polished models.
Having a lacquer/varnish over the polished metal, tarnishing can develop underneath. Maybe you can remove coating and re-polish metal surface.
You would lose any printing (registration/detailing) and most likely need polishing it every so often to keep it shiny.
Which sounds like a lot of hard work so a case of living with it?