Branches with slashes in their name (eg bugfix/issue-123, feature/abc) are not supported at the moment.
As a workaround, replace the slash of the branch name in the URL with %2F, for example, the bugfix/issue-123 branch URL https://gitlab.com/your-repo/test-files/-/blob/bugfix/issue-123/src/test.txt?ref_type=heads to https://gitlab.com/your-repo/test-files/-/blob/bugfix%2Fissue-123/src/test.txt?ref_type=heads
Let us know your use case here if you require support for slashes in branch names.