FYI I ended up installing fresh vanilla clover for the test purposes and ubuntu were loading just fine.
Then i started to add my custom plist, drivers etc one by one to track down the issue and it turned out to be FwRuntimeServices.efi / AptioMemoryFix.efi (sry, don't quite remember). I...
Yes, it is already set to true. Just updated Clover to latest 5117 and now i have two linux entries. Both however point to the same GRUB2 bootloader and if i choose Ubuntu there i get black screen in both cases.
I had a dual boot setup on separate SSDs (Catalina + Win10, both UEFI).
Today i installed Ubuntu 20.04 (also in UEFI) on third SSD, with other drives disconnected.
Now when reconnected all of my drives i can boot Ubuntu from BIOS boot menu but not from Clover:
I can see Ubuntu Linux entry...