bug#35898: 26.1; line-beginning-position sometimes does not yield line beginning position

[Markus Triska <triska@metalevel.at>]

Please download hello.png with:

    $ wget https://www.metalevel.at/ei/hello.png

and then, starting Emacs with "emacs -Q", evaluate the following form:

    (progn
      (goto-char (point-min))
      (save-excursion (insert "\n"))
      (goto-char (+ (point) (cadr (insert-image-file "hello.png"))))
      (line-beginning-position))

This yields 592, whereas I expect it to yield 1.

For comparison, when I do C-a to move to the beginning of the line, it
moves to position 1, as expected.

Is there a way to make line-beginning-position yield the line beginning
position in cases like this? Would you please consider adding this feature?

Thank you and all the best,
Markus

In GNU Emacs 26.1 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2019-04-09 built on mt-laptop
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000