Cannot see new Startup Disk after careful installation of new version of OSX

Hi all. I run OSX 10.14 on my MacBook Pro and wanted to upgrade to Monterey but only on an external SSD so that my old Photoshop CS6 applications could still be used as I could flip between Monterey and Mojave operating systems. I carefully downloaded and installed Monterey on my 1TB SSD and successfully rebooted into that version and installed some new versions of my Apps, namely Final Cut Pro X. I then switched back to OSX Mojave by selecting 10.14 as one of the 2 options in 'Select the system to start the Computer'.

I was really disappointed to see that the OSX Monterey system was not displayed in the Startup Disk panel under my Mojave system and I could see no way to reboot under the new up to date version of OSX!. I tried a reboot and reset the Non volatile memory to no effect. Any thoughts or help would be most gratefully received.

Regardf Richard