the pulp team identified a bug: https://pulp.plan.io/issues/7321 introduced in pulp_rpm 3.5. There is a patch available: https://github.com/pulp/pulp_rpm/pull/1837
if you want to try to apply it:
cd /usr/lib/python3.6/site-packages/pulp_rpm/
curl https://patch-diff.githubusercontent.com/raw/pulp/pulp_rpm/pull/1837.patch | patch -p2
systemctl restart pulpcore-worker@*