rev-parse --show-toplevel broken during exec'ed rebase?

Typically git rev-parse --show-toplevel prints the folder containing the .git folder regardless what subdirectory one is in. One exception I have found is that if one is within the context of git rebase --exec then show-toplevel always just prints the current directory it's running from.

Repro (starting with cwd within git project):

... path to git repository

# Stop at some commit for editing

... path to git repository

... path to git repository/xdiff !!!

This seems like incorrect behaviour to me since it's a weird inconsistency (even with other rebase contexts) & the documentation says "Show the absolute path of the top-level directory." with no caveats.

Thanks, Vital