The bug

I placed a dispenser facing upwards to shoot out a fireworks rocket and the rocket exploded inside of the dispenser before launching into the air. Even with different flight durations it still went off inside the block.

I tried shooting a rocket in other directions and it works just fine.

To reproduce

1. Get exploding firework rockets

   /give @s minecraft:firework_rocket{Fireworks:{Flight:1b,Explosions:[{Type:0b,Colors:[I;11743532]}]}} 64
   

2. Put them inside of an upwards-facing dispenser
3. Power the dispenser
   → The firework rocket explodes inside of the dispenser