Hmm. Can you file an issue against rpm-ostree for this? The forum here is OK for “broader” issues but this one is clearly a bug in the rpm-ostree stack. Can you try e.g. using rpm-ostree usroverlay, then download different ostree builds and use rpm -Uvh --force, and then systemctl restart rpm-ostreed and see if that helps?
Also, if you can reproduce this with plain ostree pull that would help. Since this issue is around pulling/networking it’s also likely reproducible in a container; rather than messing with the host you could use podman and create a container, then yum -y install ostree, ostree --repo=repo init, then ostree remote add ... etc.
One other useful command here is e.g. env OSTREE_DEBUG_HTTP=1 ostree pull fedora-silverblue:fedora/29/x86_64/silverblue