To reproduce:
1. Open project "parentproblem"
2. Open scene "parentproblem"
3. Enter play mode
4. Move player controller into the cube on scene
5. When OnTriggerEnter is called the player controller is made child of the cube
6. Observe as OnTriggerEnter keeps getting called, while player is moving inside the cube
7. Observe as OnTriggerExit is NOT called when player leaves the trigger

(OnTriggerEnter/OnTriggerExit call counts can be inspected on the cube's "parentme" script)