Thanks cseevinck! I can’t download your json for some reason, but I can work out what you mean about the test setup.
I tried migrating the fields to another test site and everything was fine, including using Seamless clone fields as sub fields. The only environment I’m having trouble with is the one I need it to work on!
Out of interest, have you tried development mode on Flywheel? This prevents all caching so should prevent any conflicts with existing fields.
