Since 1.16 was released, hostile mobs are spawning in well lit rooms. Seeing Creepers, Spiders, Skeletons, etc all spawning (and at an alarmingly high rate) in rooms that have always been "secure" in the past.
In one example, I left a smallish (maybe 30x40 room), previously secure area for a few minutes and when I returned, there were no less than 2 creepers, 3 skeletons, 2 spiders and a witch all attacking me! We literally were not able to repair the damage as fast as the creepers were inflicting it... and this was EASY mode! Have had to set to Peaceful as it is now completely unplayable otherwise!
Details: XBox One console
World: Created in 1.14
Multiplayer: Yes
Mode: Survival
Difficulty: Easy
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It could be that you lit the area with soul lanterns, soul torches or soul campfires because those items cause mobs to spawn.

@@unknown that is not true. The game does not distinguish different kinds of light.
More likely is that the new despawning rules have allowed the cave cap around the base to clear, and the area is not adequately lit. In previous versions, as you spent time in an area the cave monster cap would tend to get permanently filled by monsters in tiny, sealed-off caves. Now those monsters can despawn.

Here are a couple of screenshots of a room that JUST spawned a spider... This was a world just created in 16.1 on the PC...
[media][media]
Thank you for providing a picture. Unfortunately, the room does not have enough light. Here is a map of the light levels in the block above the floor for a 10x10 room with torches 3 blocks up the wall 3 blocks from each corner, with spawnable spaces in red:
10 | 11 | 12 | 11 | 10 | 10 | 11 | 12 | 11 | 10 |
11 | 10 | 11 | 10 | 9 | 9 | 10 | 11 | 10 | 11 |
12 | 11 | 10 | 9 | 8 | 8 | 9 | 10 | 11 | 12 |
11 | 10 | 9 | 8 | 7 | 7 | 8 | 9 | 10 | 11 |
10 | 9 | 8 | 7 | 6 | 6 | 7 | 8 | 9 | 10 |
10 | 9 | 8 | 7 | 6 | 6 | 7 | 8 | 9 | 10 |
11 | 10 | 9 | 8 | 7 | 7 | 8 | 9 | 10 | 11 |
12 | 11 | 10 | 9 | 8 | 8 | 9 | 10 | 11 | 12 |
11 | 10 | 11 | 10 | 9 | 9 | 10 | 11 | 10 | 11 |
10 | 11 | 12 | 11 | 10 | 10 | 11 | 12 | 11 | 10 |
This ticket will be resolved as duplicate of MCPE-302.
Can you provide a picture, video, or world save to show the area where monsters spawn unexpectedly?