A number of hostile mobs (creepers, skeletons, spiders, slime and endermen) have been spawning inside of my well lit underground base/mine. I had no issues with hostile mobs in these same areas before the update. The areas are between Y10 and Y23. I've added an additional stack of torches around the area and mobs are still spawning.
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screenshot now attached.

From the picture, it looks like your light levels are not high enough. For example, counting from the torch above the chests on the right of the picture, the block in the middle of the chiseled stone doorway is at light level 6. Monsters spawn in light level 7 and below. Most likely what kept monsters from spawning in your base in the previous version was the cave monster cap being filled by monsters hidden in small inaccessible caves. With the new despawning system in 1.16, those monsters are now able to despawn regularly, so new ones can spawn in other places.

Thanks for your help

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We're tracking this issue as MCPE-302, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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Could you include a screenshot or video of the area?