Recently (around November 2022) custom field extensions are no longer applying to fields.
Following the guide of npx create-react-app@latest my-first-plugin --template datocms-plugin --use-npm and adding a simple plugin to change the UI of a JSON field inconsistently works.
Thank you so much for the very detailed report!!
I was able to reproduce the issue exactly as described, i’ll get back to you as soon as the issue is solved