This is an FYI post for anyone who has the same issue that I have finally solved.
My Roon Core became unstable and kept repeatedly crashing.
Investigation showed memory usage steadily increasing until it ran out and the machine froze up.
Spent ages trying to diagnose including rolling back to older Roon versions, restoring old databases, even spinning up a fresh Roon core with empty database - on the assumption it was a Roon core or database issue - but still kept happening no matter what.
Long story short I eventually noticed that despite still working fine with the KEF app, my KEF LS50 Wireless II speakers had disappeared as a Roon Ready device in Roon (but were still appearing as an Airplay device).
In the end, unplugging the KEFs from power and then restarting them cured the issue - so clearly the KEFs were to blame.
Moral of the story I have taken away is that an endpoint can screw up your core. So in future I will do a hard reset on all my endpoint hardware when trying to diagnose issues.
Hope this saves someone the pain I have just been through!
Roon Core in Ubuntu 20.04 container on Proxmox VE
Bluesound Pulse x2
Bluesound Node II
KEF LS50 Wireless II