I think this may related to the failure seen in https://github.com/pantsbuild/pants/runs/8173305790?check_suite_focus=true

✕ src/python/pants/base/specs_integration_test.py:specs_integration_test failed in 80.01s.

Also checked locally: on main:

% ./pants list not-a-source-root/slow.py
not-a-source-root:bin
not-a-source-root/slow.py:lib

on release branch:

% ./pants list not-a-source-root/slow.py
not-a-source-root/slow.py:lib

Filed https://github.com/pantsbuild/pants/issues/16772