Sam, are you still suffering from this issue? Do you have a custom environment you're working with, or just the default arcgispro-py3 environment? There are numerous places that Python can be injected into the environment that aren't entirely obvious (system variables, registry keys, cross-linked .pth files) so debugging can be tricky. Can you reproduce this from a command line session of Pro via the Python Command Prompt session? I can give you some starting points before you jump on with support.