Mmm, COPR and Koji I think use the same mock configs but someone else would have to confirm that :)

It's definitely possible that one of a packages build dependencies pulls in something that a package /should/ depend on and then when it drops it suddenly the build fails. That's why I think we mostly want 'autogenerated' BuildRequires nowadays that are generated based on the upstream source or packaging format to make that less of a figuring out excercise.