I have all my farms underground, food,fungus, cactus/cane,animals and slime. For some reason, the one with food plants, the light level on F3 registers as 0.

0 from sky, 0 from blocks.

The other areas all have pumpkins too but they register. Some are above the farm, slime farm is below.

As you can see from Outside view picture, all the levels are lit up but the plant farm in light level wrong picture, shows as zero.