It is not possible to switch to flight mode when the player is in sneak mode, 2 blocks high.

This bug has been present since release 1.20, and is still present until beta 1.20.
Bug: When a player (located in a place with a height of 2 blocks) enters crawling mode, the player will no longer be able to switch to flight mode. But in the same conditions (the player, in a place 2 blocks high) when the player is not in the mode
sneaking up, then he can easily switch to flight mode.