Git commit 233c91d32a. Has been broken at least for a few days: my previous build also exhibited this.
No idea how to create a short test in Elisp.
From user point of view: open a Magit buffer in a directory with at least two locally modified and uncommitted files. The buffer shows sth. like this:
...
Staged changes (N)
modified file1
modified file2
...
Move the point with C-n to the line with `file2'. Press TAB. Magit section for the file is not opened. Use `M-x describe-char'. From the resulting buffer you can say that the point is actually on an invisible char '@', i.e. start of (hidden) diff for the first file.
It may be necessary to restart Emacs if you have used Magit in this directory previously.
This used to work fine (i.e. point was never on an invisible character) both in Emacs 28 and earlier snapshots of `master'.
Paul