Preflight fixup 'spot colors to cmyk' messes up transparency

I made a preflight droplet with a fixup that converts a PMS color (Pantone 361) to a specific cmyk value. This is the setting:

I’ve learned the hard way (after the document was printed) that this fixup struggles with transparency.

You can download the droplet containing the fixup here.

Any ideas to fix the fixup so it handles transparency well?