Emacs version: 26.2
Steps to reproduce:
emacs -Q
Activate display-line-numbers-mode
Insert some word at the first line, like "aaa"
Execute (nth 2 (posn-at-point 2)), it will return (40 . 0)
Execute (put-text-property 2 3 'display "A")

Execute (nth 2 (posn-at-point 2)) again, it will return (4 . 0)

This breaks next-line/previous-line in cases display property is used. For example, highlight-indent-guides mode uses display property to draw indentation levels.