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.