Ah - I know what it is!
You likely added your own Blender version variant to the application settings?
You are most likely facing a a Blender “feature” where it doesn’t include PYTHONPATH passed along to it by default.
In the application settings, for your variant - expand the Arguments section.
In there add:
--python-use-system-env
As a comparison you can check the argument sections of any default pre-defined application variants for blender. Without that argument, Blender will force exclude everything from PYTHONPATH from its sys.path and hence does not include any external python libraries.