although I don't have an explanation for you, I can share my experience. Just a couple of hours ago I tried to register a SLES15 VM to SCC, so no RMT is involved. However, my VM added the respective repository and was registered, but I got this error message:
sles15:~ # SUSEConnect -p PackageHub/15/x86_64 -r <CODE>
command 'zypper --no-refresh --non-interactive install --no-recommends --auto-agree-with-product-licenses -t product PackageHub' failed
Error: zypper returned (106) with 'Error building the cache:
[SUSE_Package_Hub_15_x86_64:SUSE-PackageHub-15-Standard-Pool|https://updates.suse.com/SUSE/Backports/SLE-15_x86_64/standard?...] Valid metadata not found at specified URL
Some of the repositories have not been refreshed because of an error.'
Nevertheless, the repo was there. And after a 'zypper ref' I could use that repo to install a package. To me this looks very related to your description, unfortunately I don't have an explanation.