Before addressing this as a potential duplicate, I tried searching for this bug being reported in Java edition under all versions, and nothing relevant found.
On the wiki, it states: "Mycelium reverts to dirt when covered by one of the light-impeding blocks above and the light level at that block is below 4. The death and spread behaviours are checked when a random tick lands on the block."
As shown on the picture, it doesn't work. This was tested both on a server and in Singleplayer in Java Edition 1.19. Settings here: 200 tick speed. Nothing happened for 5 minutes. This has NOT been tested in 1.19.2. Please ignore the version I selected. No other versions were shown for me to select.
I tried using a method that SilentWhisperer showcased a few years back to convert a mushroom island into grass blocks. Reference video here: https://www.youtube.com/watch?v=nFUIJwEgz5c
Yes, the video is a Bedrock edition video, however the wiki usually shows differentiates between versions, this seems to be a universal feature that no longer works.
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@Avoma
Correct, but seeing a video confirming that this is a feature one upon a time and nothing about it being removed is enough evidence to declare a feature have being in the game and no longer is.
Can you reproduce this behavior in any version of Minecraft: Java Edition? I tried this in 1.8 and 1.2.5 and the mycelium never decayed.
Darkness never killed off mycelium(as it never killed off grass either), only if the block directly above it blocks light from reaching the mycelium will it die out. That's how it's always been.
Resolving this as Invalid then. Even if this occurred at some point in Bedrock Edition, this would not be a valid parity issue.
The wiki is not a valid source when declaring if a feature/mechanic is intentional or not.