pipenv installed pip does not work with specified python version - raspbian

Question

On a Raspberry Pi OS Bullseye system, I tried to install numpy with pipenv using a specific python version and got this:
$ pipenv --python /opt/python/3.7/bin/python3 install numpy --verbose
Creating a virtualenv for this project…
Using /opt/python/3.7/bin/python3 (3.7.9) to create virtualenv…
⠋created virtual environment CPython3.7.9.final.0-32 in 410ms
creator CPython3Posix(dest=/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC, clear=False, no_vcs_ignore=False, global=False)
seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/pi/.local/share/virtualenv)
added seed packages: pip==20.3.4, pkg_resources==0.0.0, setuptools==44.1.1, wheel==0.34.2
activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator
Virtualenv location: /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC
Installing numpy…
⠙Installing 'numpy'
$ "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip" install --verbose "numpy" -i https://pypi.org/simple --exists-action w
⠙
Error: An error occurred while installing numpy!
Traceback (most recent call last):
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip", line 5, in <module>
from pip._internal.cli.main import main
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/main.py", line 10, in <module>
from pip._internal.cli.autocompletion import autocomplete
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>
from pip._internal.cli.main_parser import create_main_parser
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/main_parser.py", line 7, in <module>
from pip._internal.cli import cmdoptions
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/cmdoptions.py", line 23, in <module>
from pip._vendor.packaging.utils import canonicalize_name
ModuleNotFoundError: No module named 'pip._vendor.packaging'
Looking at the verbose output i see that the path to pip used by pipenv is /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip.
Calling this pip directly indeed leads to the same error:
$ /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip --version
Traceback (most recent call last):
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip", line 5, in <module>
from pip._internal.cli.main import main
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/main.py", line 10, in <module>
from pip._internal.cli.autocompletion import autocomplete
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>
from pip._internal.cli.main_parser import create_main_parser
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/main_parser.py", line 7, in <module>
from pip._internal.cli import cmdoptions
File "/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip/_internal/cli/cmdoptions.py", line 23, in <module>
from pip._vendor.packaging.utils import canonicalize_name
ModuleNotFoundError: No module named 'pip._vendor.packaging'
Which python is used in that case? Looking at the shebang line it would seem it's the one I passed to pipenv initially:
$ head -n 1 /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip
#!/home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/python
$ ls -l /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/python
lrwxrwxrwx 1 pi pi 27 Dec 11 11:00 /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/python -> /opt/python/3.7/bin/python3
But when I explicitly use that exact interpreter there is no error:
$ /opt/python/3.7/bin/python3 /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/bin/pip --version
pip 20.1.1 from /opt/python/3.7/lib/python3.7/site-packages/pip (python 3.7)
The difference seems to be that in the case it goes wrong, the pip installation in /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip is used while in the working case it's the one in /opt/python/3.7/lib/python3.7/site-packages/pip.
But why? My understanding of the shebang is that it points to the interpreter that's to be used. In the working example all i do is call that interpreter explicitly myself. Why is there a difference in behaviour?
And also, why did pipenv even install its own pip in /home/pi/.local/share/virtualenvs/deep-dregs-eaJke9eC/lib/python3.7/site-packages/pip ? Why didn't it reuse the pip that comes with the python version I passed? And if that's just how pipenv works, why is its pip broken? What's going on? And how can I fix it?
EDIT
When i use my system python 3.9 installation it works fine.