On bastions and several other stretch VMs reply with this kind of stuff on apt update:
E: Failed to fetch http://apt.wikimedia.org/wikimedia/dists/stretch-wikimedia/InRelease Unable to find expected entry 'thirdparty/kubeadm-k8s-1-19/source/Sources' in Release file (Wrong sources.list entry or malformed file) E: Some index files failed to download. They have been ignored, or old ones used instead.
Not sure why yet. The bastions do seem to be running the latest-installed kubectl, so this would be a recent issue.