As stated by @JulianKoster, it was just a bug and it was patched just a few hours after my question : see this link

For people struggling, just commit/stash your changes, run composer recipe:update and you will see some stimulus-bundle changes about CSRF.

After this update, form submit with csrf enabled works even from inside a turbo frame ! No additionnal code needed.

Thank you too @rapttor for your time !