There is a hardcoded 'uksouth' value for LOCATION on line 179 (in the "Deploy management" step) of the register_tre_bundle.yml workflow file.
After fixing that locally to use the correct location, running the workflow on a topic branch (and leaving prRef empty) results in the following:

The issue appears to be that the "latest" tag is hardcoded in the container image but the register action is looking for a tag based on the refid of the workflow.