Note that I have 11 apps that disable greyscale mode, but 12 apps that enable it. Why one more to (re-)enable greyscale?

This one app is: WhatsApp.

That's because I set up to fire the »enable greyscale« automation whenever I close WhatsApp. Of course, I do not enable colour for WhatsApp, but I use WhatsApp quite often, so this is a »safety fallback«: whenever colour mode remains enabled by accident, it won't endure me closing WhatsApp, which happens often enough. (I used to have a »every day at midnight, reset to greyscale« automation, and »whenever I close WhatsApp« replaced it for me.)

You might need something like this because locking your phone won't count as »closing the app«, so your phone might stay in colour mode after locking it from, say, Maps, and then be still in colour mode when you unlock the device (ending up on your home screen, if the phone was locked long enough to not bring you back to Maps anymore).