The git hook at .git/hooks/post-checkout is missing the IS_BRANCH_CHECKOUT env var. Git passes 3 args to post-checkout: prev-ref, new-ref, and is-branch-checkout (0 or 1). Update the hook to export IS_BRANCH_CHECKOUT=$3 before calling git-branchless.
Cannot reproduce: No post-checkout hook exists in .git/hooks/. git-branchless is invoked directly by git hooks mechanism without a wrapper script. The IS_BRANCH_CHECKOUT arg is passed correctly by git itself to branchless hook commands.
Closing as won't-fix/can't-reproduce.