Sorry, I was not specific enough. My 2012-era Mac mini (new in 2014) will not boot from an encrypted external USB drive. See the Bombich Known Issues page:
“2012-vintage Macs can’t boot macOS Catalina from an encrypted USB device
“We have received several reports that the 2012 Mac mini and the 2012 MacBook Pro can initially boot from a non-encrypted external USB device, but then will fail to boot from that device when FileVault is enabled on the external device. This issue is not specific to CCC, we have confirmation that this occurs when installing Catalina directly onto an external device as well. This problem does not appear to be specific to any particular enclosure, rather it appears to be specific to the 2012 models of Mac mini and MacBook Pro.”
I found out the hard way. 