Creeper with negative ExplosionRadius deals damage to Resistance effect lvl 6+ entities

When I set the creeper's ExplosionRadius to -1, it damages those who have Resistance lvl 6 or more.

I used this command to summon the creeper:
/summon creeper ~ ~1 ~ {ExplosionRadius:-1,Fuse:0}

I used these commands to apply the resistance effect:
1. /effect give @e[type=!player] resistance 100000 5 true
2. /effect give @e[type=!player] resistance 100000 6 true
3. /effect give @e[type=!player] resistance 100000 127 true
4. /effect give @e[type=!player] resistance 100000 255 true

For the 4 /effect commands above,
1. Can fully protect the mob
2. The mob took a bit of dmg (around 2.5 hp?)
3. The mob took a lot of damage
4. The mob took EVEN more damage
(Those without Resistance don't take damage as well.)

Is this intended?