Mobs spawn in Peacefull

When I tested some things in my creative amplified world,
I turned off Normal mode and put it to Peacefull, but then I noticed a skeleton who didn't dissapear. I hit it normally once, but then I couldn't hit him anymore, he also didn't shoot arrows and he couldn't move.
(Check Image 1)
The only thing I changed was /gamerule TileDrop false.
(Probally not relevant)
When I restarted the game, the skeleton was gone.
(Check Image 2)

Thanks.
Fevolmine