I did some more digging around and found Katello 4.0/pulp3: no erratas anymore
The problem seems to be identical (with the difference being SLES vs. EPEL repos).
My problematic content view was also no composite Content View, but had one filter applied. As the filter is not that important, I removed it and now the publish results in a perfectly fine CV.
I will leave the CV as is to have it working and apply the filter again after the Issue (Bug #32435: Excluding packages from EPEL 7 (at least) causes all errata to disappear - Katello - Foreman) is resolved and patched in our instance.