1.21.71 came out yesterday, and I can't understand why this bug hasn't been fixed.
This also happened to me in my survival world; the books no longer store new information. I'm on an Android 14 tablet. I'd like to add another platform where this bug occurs.
ScottishKevin, thanks for pointing out that they are actually going under the world, I'm going to change the bug description slightly, although I've already seen that this report was changed to "Duplicate" for MCPE-157125, which the Mojang team is tracking.
This bug exists in 1.21.51, and the speed effect is also applied in spectator mode. (I may make a separate report for the soul speed effect in spectator mode).
UPDATE: I make a report for the speed boost in spectator mode (MCPE-190152)
The issue continues in 1.21.71.