Mojave doesn't recognise Big Sur partition
The Startup disk utility in Mojave (10.14.6) doesn't recognise my Big Sur system and so I can't swap back to Big Sur. I used to use Startup Disk utility to swap between Catalina and Mojave all the time. There's an error message "incompatible disk - this disk uses features that are not support on this version of macOS". How can I go from Mojave to Big Sur? Thanks
iMac 27″ 5K, macOS 10.14