How to manage venvs for subprojects within a workspace?

[somiandras]

1. I have a multiroot workspace with multiple Python (sub)projects in subfolders. All of them are managed with Poetry with proper pyproject.toml files and each installed into a .venv directory in the project itself (not a central Poetry env cache).
2. I also have a "master" project in the workspace root, mostly for dev tools and dependencies to project scripts, etc. (I don't want to install all the dependencies from the subprojects into the master).
3. I used the Python Environments extension auto-discovery tool to recognise the venvs in the subprojects. Based on the extension panel it worked.
4. When I open a .py file in any of the subprojects, the status bar shows the proper environment so I would expect Pylance to pick up the correct interpreter path and evaluate imports using that, but it fails.

I created a toy example that fails in the same way. I have subproject_a and subproject_b, both has their pyproject.toml and .venv, both recognised by the environments extension (left panel). The status bar properly shows the subproject_a environment, which has polars installed, but Pylance still fails to recognise. it. What do I miss here?