Select-box appearing as text

Hi @artfulrobot and everyone else. I might be at the wrong project repo but I'll give it a shot: I am having a rather odd behaviour which I cannot understand.

The same form-processor form on 2 different sites appears properly on the form processor's preview but on the inlay/inlayfp preview it appears differently: on one site country appears (correctly) as a selector while on the other site, country appears as text.

Correct appearance:

Odd behaviour of the select (appearing as text):

I cannot understand why it's doing that or what I should be looking at so that I can troubleshoot. Again, the source of FormProcessor is identical.

Attaching the source of FP: sample_newsletter.json

Any hint could be precious!