I work constantly with a mix of both 44.1k and 48k sessions.
With C12.0.50, sessions do not reliably open at the correct rate, and as I’ve yet to find a way to force a session to change rates without Cubase wanting to change all the recorded audio, you have to close it down and re-open again.
I have never had an issue with sample rates until this version, and now I’m getting them all the time.
Hmmmm, might be related to your audio interface. Mine comes with a separate control app where you set the sample rate of the audio interface (outside of Cubase), and sometimes it changes between 44.1 and 48, and I have to manually go into that app to set it to match the Cubase sample rate (I do everything at 48). Not sure whether that’s what’s going on in your case, but thought I’d mention it.
In my case its always switched rates automatically perfectly on opening a session - I cannot remember it ever not working in the last 25 years!
However with this latest update only, it fails maybe 30-50% of the time. So I’m seeing it happen daily.