The bug

So basically, if a player where to reel out their fishing rod bobber towards block(s) above them... the game would for some reason make the bobber be stuck at OnGround:0b for a long period of time. Even if the bobber hits the ground afterwards, the bobber would still be stuck at OnGround:0b until the game decides to detect it that it's on the ground (which takes quite a while by the way).

See https://www.youtube.com/watch?v=nu-HD01nuEw&feature=youtu.be

How to reproduce

1. Cast a fishing rod towards the ground
2. Use the following command to verify that OnGround is set

   /say @e[distance=..10,nbt={OnGround:1b}]
   

   → It says "unknown", which is the fishing hook

3. Cast the fishing rod against the ceiling and make sure the hook hits it
4. Repeat step 2
   → It does not list the fishing hook