This bug affects the following projectiles:

• dragon_fireball
• fireball
• potion
• small_fireball
• wither_skull

Steps to reproduce:

1. Use the command /summon minecraft:fireball
2. Look in chat and it says "Unable to summon entity"
3. Look in the console/output log and it says "Tried to add entity minecraft:fireball but it was marked as removed already"
4. Repeat steps 1-3 with the other 4 projectiles on the list and see that it has the same result for all of them

Note: Adding coords and/or data tags to the summon command don't affect the outcome.