Okay, so, I made a new world, and I found this cave that had a light level of 0 in it, apart from the brown mushrooms that were inside. The Mushroom appeared to be producing a light level of 1. I don't know if this is intentional, but I doubt it. I was playing in creative mode, but it also works in Adventure and Survival. The First Screenshot is the world I First discovered the Glowing Mushrooms, The Second and Third Screenshot is me in another world confirming the issue. The two is me placing down the Brown Mushrooms in both Adventure mode and Survival mode ti prove that the issue works on all three game modes not including Spectator mode.

To Recreate:

1. Load a world in version 20w21a.
2. Enter a cave or create a structure where the ambient light level is 0.
3. Place a Brown Mushroom on a block.
4. Observe how it produces a tiny amount of light.