@RandC
I ran into the same issue on my Z390 Designare recently when I decided to replace my Samsung 970 EVO because of the "slow boot" issue with Samsung drives and Monterey. I used the current version of Carbon Copy Cloner to make a bootable clone to an external USB-connected SSD. That booted without issues. I then replaced the 970 EVO on my Designare with the new SSD. I formatted it, and used CCC to clone the external drive to the new SSD.
I forgot to install my EFI folder on the new SSD. Booting with the Delete key into BIOS, the only boot partition displayed was the Windows Boot Loader on my second internal SSD. (If I recall correctly, even the external SSD used for cloning now wasn't displayed in BIOS or with the Fn12 key.) I was shocked! Luckily I had a USB thumb drive that I could use to boot into the OC 0.7.5 picker via the Fn12 key, and access the newly installed internal SSD. I got the EFI folder installed on the new SSD on the board. But it still wasn't displayed in BIOS as bootable.
I'm sure the CCC cloning was the root cause. A wild guess is that the drive was not "blessed?" But I don't know how that works.
I spent several hours trying to resolve the problem. I de-powered and shorted the board's BIOS reset pins. BIOS then displayed all my drives, including the new SSD, but
the only one that showed the UEFI for booting continued to be my Windows Boot Manager. I continued to use my USB thumb drive for booting into the OC picker to get to my new SSD.
Unfortunately I don't remember what sequence of events solved the problem! I think I had to reinstall Big Sur on the new internal SSD to finally get the UEFI folder recognized in BIOS and bootable. (You have already re-installed Big Sur, so that doesn't work for you.)
Sorry I don't have a solution for you, but I can confirm I had a similar issue involving CCC doing a "merry-go-round" clone.