A quick search leads to hundreds of cases of problems with branch name case
including one from atlassian where there are problems with mixed case in
branch names:
https://confluence.atlassian.com/display/BitbucketServerKB/Case-sensitivity+in+branch+name+causes+issue

Microsoft recommends all lower-case branch names:
https://docs.microsoft.com/en-us/azure/devops/repos/git/case-sensitivity?view=azure-devops#branch-and-tag-names

Some git clients will even convert some parts of branch names to
lower-case, like when using '/' in the name. GitHub Desktop had problems
with this, Visual Studio had problems with this, Jetbrains products had
problems with this, etc.

There are lots of documented problems for GitHub, GitLab, Bitbucket and
other providers about mixed case branches and tags.

For all these reasons, most git guides, naming conventions, etc recommend
using all lower-case branch and tag names.