So net-net does this mean it’s behaving as expected,

It is as expected.
If no "Installed Packages" are listed, then it cannot pull a cached venv (because requirements.txt is not a full env, and it never analyzed it)).
It does however create a venv cache based on it (after installing it)
The Clone of this Task (i.e. right click on the UI clone experiment, enqueue it, Will use the cached copy becuase the full packages are listed in the "Installed Packages" section of the Task.
Make sense?

BTW:

it shouldn’t be spending 2 mins “installing” already cached/downloaded pkgs.

unfortunately pip is not very efficient in installing packages, even if they are already downloaded, hence the cache in the first place