Summary:
Almost simultaneously pressing (holding) the shift and jump turns on/off the flight mode. You do not need to press shift and jump at the same time, 1 second between pressing shift and jump is enough. The order in which the buttons are pressed does not matter. If you were in flight - the flight will be disabled and vice versa. Also, if you wait more than ≈1 second between pressing the buttons or hold down the buttons at the same time, the flight mode will not change.
This bug was not in 1.18.12 (and earlier versions), and also this "feature" does not correspond to the Java version.

Steps to reproduce:

1. Switch to creative game mode.
2. Turn on flight mode (by double tapping jump).
3. Hold shift and hold jump after ≈1 second.
4. Hold shift and hold jump after ≈1 second.

Observed results:
Almost simultaneously pressing (holding) the shift and jump turns on/off the flight mode.

Expected results:
Instead, the player must shift jump or shift while in the air, as if holding shift and jump at the same time, or waiting more than ≈1 second between button presses.

Video:
Minecraft Preview 2022-03-14 12-52-26.mp4
1.18.12 and below
Minecraft 2022-03-14 13-38-23.mp4