Having the latest master code (commit e9cc07b) on copr-fe-dev and copr-be-dev, I tried to fork @copr/copr-dev - and this was the actions.log:

We are copying the latest successful Build per package, not BuildChroot per CoprChroot. So instead of trying to copy what really is the latest used RPM, we try to copy builds which are already obsoleted and garbage collected.

I believe this is now correctly fixed in rawhide_to_release command, after commit 76dbc56 -- so we should probably use the same approach for picking buildchroots for forking as well.