What I think it was going to do

I expected that mobGriefing would work with the the small_fireball no matter the entity type of the Owner similar to the large fireball.

What actually happened

When a small_fireball is summoned with motion that then collides with a block it will set fire to it even if mobGriefing is set to false unless the Owner tag is an entity that isn't a player, projectile, armour stand or the is UUID of a non existent entity in the world (There maybe more cases of this but these were the ones I tested).

How to Recreate

1 - (Only needed if you want to test if the fireball griefs with certain entities) put the command "execute as @e[type=small_fireball] run data modify entity @s Owner set from entity @e[type=<entity>,limit=1] UUID" in a repeating command blocks that is always active (replace <entity> with any entity/tag you wish to test, the entity must also exist in the world).

2 - Turn off mobGriefing with "/gamerule mobGriefing false".

3 - Use the command "/summon small_fireball ~ ~ ~ {power:[0.0,-1.0,0.0]}" (power can be replace with Motion for the same result) while you are above a block and let the fireball collide with it.

Result - The block will be on fire (Unless you used a compatible entity (like a blaze) in the first step)