Summary:
In creative and if the player is flying. The game does not properly register the new state of the player when the player exits a rideable entity. This causes the game to "think" the player is still flying while walking. Causing the player to repeatably fall from the sky or be re-positioned.

Steps to reproduce:

• Make sure the player is flying in creative
• Click on a rideable entity like a minecart / boat / horse (to ride the entity)
• Exit the entity (jump single press or shift)
  note: Do not press jump twice (or repeatably) this can cause the player to enter the flying state again
• Move up to a higher altitude by holding jump or climbing blocks (jump single press) or move to a different position (again be careful to not press jump to fast)
• Notice the player position might reset while moving or the player appears to be falling from the sky.

Double tapping jump resets the issue. The player enters flying mode again.

Video attached (example):
Minecraft 2019-09-29 18-41-31 clip.mp4