The good news: Anki’s Frozen Fields does what it’s supposed do, but:
when I first invoke the Add dialogue, some(!) of the pins are greyed out, even though the field is sticky (as I want it to be).
Clicking the pin twice now shows the status correctly, i.e. active.
Other fields show the status correctly from the beginning.
In case this is relevant: the note type has 15 fields.
May or may not be related:
the Add-On Collapsible Fields also shows some erratic behaviour; i.e. for some fields, the Add-On works as supposed (field collapsed by default), for other fields not.