• Mushroom worksites do not harvest or plant mushrooms/netherwart.
• Torque power seems to connect but no 'work' is accomplished.
• NPC Farmers do not interact with worksite.
• All other worksites seem to function properly.

Mushroom farm follow same block placement rules as a player.
Nether wart goes on soul sand, mushrooms in low light.

I do not understand your first comment about player block placement.
I will try to replicate the issue but I did some extensive testing....
I understand the requirements to grow mushrooms.
I planted the farmed area full of mushrooms and they would not 'harvest' either if planted manually or grown 'naturally'.
I could not get the farm block to plant mushrooms under any circumstances.
The farm block appears to accept 'power' through a shaft but no work (ie; plant or harvest) is ever accomplished. NPC farmers will not interact with the block.