I started a singleplayer world on this seed 932548120547947. In forge 1.14 it spawns with huge mushrooms and with every version of 1.15.1 forge it doesnt. Also i was not able to make huge mushrooms by bonemealing a mushroom in a 1x1 pit on mycelium like wiki says. This is a screenshot of the world with forge https://imgur.com/a/wHrqXzt
this is a screenshot of the world in vanilla 1.15.1 https://imgur.com/a/iYq82AX