Ok, this is what happened:
I made new definitions (save, apply) and tested them. They worked. I did closed calibre. On reopening calibre and reusing the definitions they did not seem to work anymore.
I checked the new definitions, but on reopening the Metadata plugboards editing window, I discovered that somehow those new definitions all disappeared and my original single definition "anyformat:any device" reappeared, but not with its original template, but with a template of one of the new definitions.
I tried to recreate the problem by defining the new definitions again from scratch, but no luck so far.
Go figure.