This is a Why question rather than a How...
I'm working on a request to require certain fields for certain issue types in specific (Company Managed) projects. The very first thing I did was to create copies of an existing Field Config and Field Config Scheme.
While working on the (copied) Config, I decided to clean up the many screens that showed as associated. I assumed this was safe because these copies were not associated to any projects. Within minutes I get a call saying the field has disappeared from screens.
I restored the screens and all is well, but if I'm working in a separate config that is not associated to any projects yet, why did this happen?
Thanks for any insight you can share.