Random mobs spawning in the air,fully light up caves and areas that they wouldnt other be able to spawn

Put the summary of the bug you're having here

What I expected to happen was...:
Was playing survival and i was strip mining making sure everything was light

What actually happened was...:
a random which spawned at y level 12 in a extremhills+ biome and then once i resoponded i got my stuff went back to our little hut inside a mountain where creepers proceeded to fall on us then after giants spawn after that some ghast's spawn in the air and finaly after we killed them a wither had spawned on us

Steps to Reproduce:
we were playing survival for about a 2 and half hours *** Also idk how to recreate it seem to stop once i was put into creative****
1. Put a step by step guide on how to trigger the bug here
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