I don't think this is documented, but when I asked about it was told by an Apple employee that this is a result of caching behavior. Copying it to a new location and then back dirties the cache and allows codesign to work. I still think this is a bug that should be fixed (at a minimum provide better messaging), but that Feedback remains open.

Thanks for you answer. I did my best to follow your advice in filing FB8968653.