The bug

If you right click with a glass bottle on the dragons breath particle and there is already a dragons breath bottle in your inventory, the new dragons breath bottle that is created stacks up with the other one. This does not happen when you use the last glass bottle in a stack.

How to reproduce

1. Summon an ender dragon with a custom UUID

   /summon ender_dragon ~ ~ ~ {NoAI:1b,UUID:[I;1,2,3,4]}
   

2. Summon an area effect cloud with the ender dragon as owner

   /summon area_effect_cloud ~ ~ ~ {Duration:2000,Radius:5f,Owner:[I;1,2,3,4]}
   

3. Right click the area effect cloud with at least two glass bottles
   → You will notice that the dragon's breath item created by the last used glass bottle is not added to the existing stack