Tags found in the packed-refs file that can be created by the Git command git pack-refs do not have to be UTF8 encoded. In these cases, GitPython fails to read these tags due to assuming they should be UTF8 compatible.

The source for this issue is found here:
https://github.com/gitpython-developers/GitPython/blob/main/git/refs/symbolic.py#L124

with open(cls._get_packed_refs_path(repo), "rt", encoding="UTF-8") as fp:

A working example (using a repository with a non-utf8 tag):

git clone https://github.com/ACRA/acra
cd acra

Create a Python script with the following content:

import git
repo = git.Repo(".")
print(repo.tags)

Execute the script:

python script.py

Result:
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc3 in position 6216: invalid continuation byte