The bug

When hooked onto an entity, fishing bobbers usually have their OnGround nbt set to 0b. However if a bobber hooks onto an entity by first touching the ground and then attaching, it will frequently have its OnGround nbt set to 1b, despite not touching the ground. 

How to reproduce

1. Give yourself a fishing rod
2. Spawn an entity that can be hooked (like a cow)
3. Cast the fishing rod at the ground near the entity so that the bobber first makes contact with the ground and then attaches to the entity
4. Type

   /data get entity @e[type=minecraft:fishing_bobber,limit=1] OnGround

   →  If done correctly, the command will output 1b instead of 0b