Yeah, not only the outdated Ubuntu 18 was causing confusion, but also some inconsistency in other places like e.g. it wasn’t predictable where Netdata got installed across different platforms, which made automation with Ansible pretty difficult.
I now went at length to completely wipe Netdata from all hosts and start from scratch by going to static builds. There I discovered another bug (see [Bug]: Netdata updater doesn't update to 1.42.1 · Issue #15851 · netdata/netdata · GitHub) but now things seem to be resolved.