Belladonna and Mandrake plants don't show any growth percentage on farmland nor do they advance in growth. Yellow Dock, Sage, and Mugwort do not have this problem and grow fine.

Belladonna and Mandrake have this issue in Covens where I found them as well.

I took some plants from a Coven spawn I found back to my house and planted them in my garden for easy access. The Yellow Dock, Sage, and Mugwort all grow without issue, and can be harvested as well. Belladonna and Mandrake are perpetually at the same stage and do not advance in growth. I observed this both at my house and when I went back to visit the coven where I found them as well. They don't show a growth percentage when hovering over them at all.