Pressing ascend & descend at the same time causes player to stop flying

Steps to Reproduce:

1. Go into fly mode in creative mode using touch controls;
2. Try crouching in mid-air by pressing both ascend and descend at the same time.

Observed Results:
Fly mode is disabled, making it difficult to use fly crouch to build.

Expected Results:
Ascend & descend buttons are counted separately for the double tap action, so fly mode is disabled either when pressing ascend twice or pressing descend twice.

Notes: