Why are collision groups bugging for me?

I've added two collision groups, one for the mobs, and one for the players, and set them to be unable to collide. This works like 80% of the time, but this happens sometimes: https://gyazo.com/e883406a2fb7631225257f34e485e1c7

I've tried manually making the collision groups and setting all of the properties, parts, etc beforehand, and I've also tried setting them in a script. Right now, I have it in a script, and I was wondering what was causing this.

Server Script in ServerScriptService:

local PS = game:GetService("PhysicsService")
PS:CreateCollisionGroup("players")
PS:CreateCollisionGroup("mobs")
PS:CollisionGroupSetCollidable("players","mobs",false) PS:CollisionGroupSetCollidable("mobs","mobs", false)

function NoCollide(model, group)
    for i,v in pairs(model:GetChildren()) do
        if v:IsA"BasePart" or v:IsA"UnionOperation" then 
            PS:SetPartCollisionGroup(v, group)
        end
    end
end

game.Workspace.Mobs.Sloobers.ChildAdded:Connect(function(Sloober) --Sloobers and Mobs are both folders
    NoCollide(Sloober, "mobs")
end)

game:GetService("Players").PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(c)
        NoCollide(c, "players")
        c.ChildAdded:Connect(function(child)
            NoCollide(child,"players")
        end)
    end)
end)

while wait(3) do --Spawning
    if #workspace.Mobs.Sloobers:GetChildren() < 15 then
        local Sloober = game.ServerStorage.Mobs.Sloober:Clone()
        Sloober:WaitForChild("HumanoidRootPart").Position = Vector3.new(math.random(0,30), 0.25, math.random(0, 30))
        Sloober.Parent = workspace.Mobs.Sloobers
        wait(3)
    end
end

To confirm that the groups are correct, I clicked on the mobs and player in the collision group window found in "Model", and the parts corresponded perfectly, meaning that they should work...

https://gyazo.com/03ad22382eeafbc6804b0c279c628aa8

https://gyazo.com/3dc9628b57eaeaab98e1a06f103c7e0a