First of all, dying in Extreme Mode enters Spectator Mode, at this time exiting the save is re-entering the save, you will find that Spectator Mode cannot fly, and will die like Survival Mode, you will not take damage after death respawns, but you will still not be able to fly, and double-clicking the "W" key to enter Sprint Mode will cause a short chunk reload
The bug will fire again when re-entering the archive