Generally, if you would like to test changes by a scratch build, you could just run fedpkg scratch-build --srpm without pushing to dist-git, whatever a forked repo or the upstream repo.

Thanks, usually I just use koji directly for scratch builds (after doing a few local builds of course). This was more a FYI about the forked repo failure.

IIRC, there are actually some issues relative to build from a forked package repo. In most cases of local build, scratch-build --srpm should work, however build in Koji doesn't. What I learned from the checkout.log, Koji should work with fedpkg together to support build from a fork repo as it seems there is no information passed to Koji so that Koji knows to clone repo from a fork repo. I have no idea what's the opinion of Koij team about this kind of build. But, it should be worth to discuss it.

IMO having fedpkg capable of scratch building from a fork repo, like COPR is able to do, would be really useful.
In my case, I forked the libreoffice repo and I need to do some scratch builds in Koji (since COPR cannot handle rebuilds on s390x because the package is too big). Yes, I could upload the srpm every time, but that means uploading ~700MB of srpm, while sources are instead available in dist-git...