The bug

When showing a text component with "underlined":true in the chat it will only be underlined if it is the last line. Otherwise the line is only shown if the F3 screen is open or if text is currently shown in the action bar.

How to reproduce

1. /tellraw @s {"text":"This should be underlined\nnext line","underlined":true}
   

   The first line is not underlined

2. Open the F3 menu or scroll to an item in your hotbar
   Now the first line is underlined as expected