Python -m venv installs a version of pip that is not upgraded. How can I upgrade it?

I often recur to the -m venv option to isolate my working environment. Python has been installed using pyenv.

However, since I have migrated to Ubuntu, I noticed a strange behaviour with the Pip command not being upgraded to the most recent version. This is strange; if - in a brand new terminal - I run something like pythom -m pip -V to date, I get the 21.3.1 version. A pip install --upgrade pip confirm pip being updated to the most recent version. That said, immediately after creating a new virtual environment, the version is not the same.

I think I have looked through all the possible sources where the Pip might be, and all of them seems to be upgraded. So my question is, from where is this stale version retrieved?

Thanks

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