Unfortunately if the external drive has a problem booting, the computer will silently decide to boot from the internal drive. The version of macOS should have no bearing on whether it works since a 2014 Mini is able to use up to macOS 12.x Monterey.
The easiest way to tell which drive you have booted from is by using Disk Utility and checking the mount point of the System & data volumes of both drives. If you are successfully booting from the external SSD, then the mount point of the system volume of the external drive should be listed as "/", while the "Data" volume mount point should be listed "/System/Volumes/Data". For those volumes on the internal drive, they should be mounted under "/Volumes" if they are mounted at all.
If you happened to clone the original drive to the external, then I have seen macOS get confused and access items from the old drive when launching apps from the external links in the Dock.
If you know for sure your external is working fine and you have a good backup of all your data, then boot to a macOS installer (Internet Recovery Mode or USB) and erase the internal drive. You will definitely know after that whether your external SSD is working properly when you reboot.