The bug

Written books that have text with underline formatting appear to lose said formatting when subtitles fade away in-game, and reappear when new subtitles show up.

Recorded behavior in action

How to reproduce

1. Enable subtitles in options
2. Create new world
3. Run command

   /give @p minecraft:written_book{title:"Underline Bug",author:"akoi",pages:['{"text":"test","underlined":true}']}
   

4. Open book and watch when subtitles fade

Notes

Update as of 20w06a:
World must be in multiplayer mode in order to open a book and read without pausing the game environment; this wasn't mentioned previously, my bad.