Neither Magit nor ‘git-commit‘ fiddle with point in the buffer used to write commit messages, so something else must be doing it.
You have probably globally enabled a mode which does restore point in file-visiting buffers. It might be a bit surprising, but when you write a commit message, then you are actually editing a file.
So you have to figure out which package is doing.
session are likely candidates. These snippets might help:
(setq session-name-disable-regexp "\\(?:\\`'\\.git/[A-Z_]+\\'\\)")
(when (or git-commit-mode git-rebase-mode)