Reproduction steps:
1. Open attached project "Collision" (Attached in the edit)
2. Open scene "Main"
3. Play the scene
4. Move off the platform and jump (Space) without touching arrow keys (Attached a gif)
-- Notice that in the console it says False - meaning that collision with the ground is not detected
5. If you move around with arrow keys, after some time it will start detecting the ground again
-- If you play further and keep braking the collision - eventually Player will start spazzing out and fall thru the plane. (Second gif)