TL;DR:

Install notebook in the virtualenv:

pip install notebook

• Snakemake apparently uses the executable notebook to start the notebooks.
• If this executable is installed outside the virtualenv, the kernels are not started from within the virtualenv
• This particular issue may be due to different versions of snakemake being used inside and outside the virtualenv.
• In fact, in the virtualenv: snakemake==8.25.3, in the user site-packages: snakemake==8.8.0