I placed my Sea Lanterns as show in the picture (gap of 13 since my light source has a value of 15). That being said my middle blocks should have a light value of 8 so no mobs should have spawn but then a Skeleton on this sample spawn you can still see the exp. and bone drop.
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Thank you for including a carefully analyzed and marked picture. Unfortunately, your calculations are not taking into account that it is the sea lantern blocks themselves that are at level 15, so the space above them is at level 14. Down the line, the light level in each space above the floor is actually one less than the numbers in your picture. The spot in the middle where the skeleton spawned is at light level 7. For more information on calculating light levels see the Minecraft wiki.
This ticket will be resolved as a duplicate of MCPE-302.
Similar issue on PS4.