Structures that replace the glowing berry wich contains the light source, leaves behind the light
This probably affects older versions, but it is clearly shown in the newest version (21w10a) with the addition of the generation of the lush caves.
Use the single biome option with lush caves biome.
The seed I could find it: 2088008021882921034
and the coords: /execute in minecraft:overworld run tp @s 271.47 23.00 1556.23 674.41 0.91
if you place a block in the center of the hole, you will remove the light source, and the light is gone