This creeper just spawned in the doorway, which should have enough light to not makes mobs spawn.
Recently, after the nether update which apparently changed mob spawning, my friend and I have been encountering problems with lighting. My friend and I were building a theatre in survival mode. On a world from earlier this year, we also built a theatre in survival, and for the seating, we lit it up with Redstone torches as we didn't want a lot of light there during the performance. This time, when we lit up every part of the seats with Redstone torches, mobs still could spawn. It was like the redstone torches didn't affect the lighting at all. Then, we decided to put regular torches underneath each stair block, however in the three blocks of space between the stage and seats, there was a little area of shadow, somehow this was enough room for mobs to spawn, even though it was not that dark. There is an upstairs area which we have lit up with torches yet still seems to spawn a lot of mobs. I will attach pictures of both areas I just described. I understamd that mob spawning needed to be fixed, but it feels like mobs just disregard any light, or spawn in an area if it has just the tiniest bit of darkness.
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Unfortunately, the areas shown in your pictures do not have adequate lighting to prevent monster spawns. For information on calculating light levels see the Minecraft wiki. When assessing your lighting, you may find it helpful to temporarily turn off smooth lighting (in video settings) to see how the light levels change block by block.
Do note that Redstone torches are not intended to prevent monsters spawns at all, as they have a light level of 7 and monsters spawn in light level 7 and below.
The reason you did not have these isseus in previous versions of Minecraft is that it was common for the monster caps to be filled by monsters hidden in small, inaccessible caves, as well as pools and rivers. The chance of this happening would increase as you spent time in an area, for example building a base or exploring and lighting the visible caves. With the new despawning system in 1.16, the hidden monsters are now able to despawn regularly, so new ones can spawn in other places. Monsters should now regularly respawn on full blocks in places you have already visited but have not fully lit.
Mobs do spoken in "tiny bit of darkness" - specifically any block with light level 7 or below. This is correct, intended behaviour.