The "Actual branch" field (actual_branch variable in LayerBranch model) cannot be blank currently. This is an oversight and was absolutely fixed in a local change that somehow got lost and not submitted. The intent of exposing the 'actual_branch' variable was not to _require_ a value, but to _enable_ a value when needed/desired. This is simply a bug.
https://patchwork.yoctoproject.org/project/yocto/patch/20240131000935.283663-1-tim.orling@konsulko.com/
Merged: https://git.yoctoproject.org/layerindex-web/commit/?id=2825a8b721c910fd20c7241df07a9989575bc322 And deployed.