bug#47893: 27.1; `display-line-numbers-mode`, line numbers hidden with multiline overlays

[Eli Zaretskii]

> From: Daniel Mendler <mail@daniel-mendler.de>
> Date: Mon, 19 Apr 2021 16:46:16 +0200
> 
> When putting an overlay over multiple lines, the line numbers of
> `display-line-numbers-mode` are hidden.
> 
> (defun bug ()
>    (display-line-numbers-mode)
>    (goto-char (point-min))
>    (let ((ov (make-overlay (line-beginning-position)
>                            (save-excursion (forward-line 4)
>                                            (line-beginning-position)))))
>      (overlay-put ov 'invisible t)
>      (overlay-put ov 'before-string "line1\nline2\nline3\line3\nline4\n")))

What do you mean by "hidden"?  With the above recipe, I see all the
line numbers starting from 5, as expected.  What is "hidden" here?