There isn't an issue.
Shroom Thumb hasn't been modified at all and it is supposed to be enabled.
The ability on Herbalism we disabled is Green Thumb, it is not needed because Magic Hoes replant.

I think the issue @FuriuosGeorge might be having here is that he wants to plant mushrooms on dirt, not activate Shroom Thumb and consume mushrooms to make mycelium.
But its not a bug, its just Shroom Thumb working as intended.
The solution is to only to plant mushrooms on mycelium or grass block, not on dirt. Shroom Thumb only comes into play when you try plant mushrooms on dirt as its function is to make dirt into mycelium.