When summoning a custom mob in survival mode all entities in the world will freeze besides yourself. There will be no gravity for the entities and you can not attack any mob. This will only happen in 1.12. I tested this with mc 1.11 and 1.10 and it worked fine. I was using mcstacker.bimbimma.com to summon my custom mobs and I was using the 1.12 command option.

I used this command for the mob:/summon zombie ~ ~1 ~ {CustomName:"1.12 Bug",CustomNameVisible:1,ArmorItems:[

{id:"minecraft:iron_boots",Count:1b}

,

{id:"minecraft:iron_leggings",Count:1b}

,

{id:"minecraft:iron_chestplate",Count:1b}

,

{id:"minecraft:iron_helmet",Count:1b}

],HandItems:[{id:"minecraft:iron_sword",Count:1b,tag:{ench:[

{id:16,lvl:1}

]}},

{id:"minecraft:shield",Count:1b}

],Attributes:[

{Name:generic.maxHealth,Base:100}

,

{Name:generic.movementSpeed,Base:.1}

,

{Name:generic.attackDamage,Base:5}

,

{Name:generic.followRange,Base:10000}

],Health:100.0f}