@arubislander I solved the problem.
Despite reinstalling my phone entirely from the UBports installer, the issue persisted.
I tried to see if I could at least trace back the issue further... until I spotted somewhere in the logs: "For interaction with a target device you might want to specify the device architecture via --arch."
I forgot to specify the architecture.
Needless to say, adding --arch arm64 to the command fixed the problem. Sorry for taking your time for such a trivial issue.
I believe it would be good to have some mechanism in place to warn me if I attempt to install a .click with the wrong architecture. It only said "failed to install" from the command line, and installing manually though the phone only gave an "incorrect package format" from the OpenStore.