The bug

In singleplayer, opening a book which is displayed on a lectern does not pause the game, but opening the book in the inventory pauses the game (MC-139508).

The behavior should possibly be consistent. Ideally both actions should not pause the game because at least for the lectern it is required to use its redstone functionality.

How to reproduce

1. Set up something which indicates that the game is running, e.g. a redstone clock
2. Open a book in your inventory
   → The game is paused
3. Put the book on a lectern and open it there
   → The game is not paused