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
Set up something which indicates that the game is running, e.g. a redstone clock
Open a book in your inventory
→ The game is pausedPut the book on a lectern and open it there
→ The game is not paused
WAI(sh). As mentioned, lecterns have side-effects, so they cannot pause, while we have no plans for changing legacy hand-held book behavior.