Some plants are not graphically picked, so the switchnodes are either not implemented or do not work as intended. This might also be due to an issue in the most recent versions of MWSE (I got a similar error for another mesh which worked fine before, but now doesn't, which points towards this). Either way, this should be fixed. The affected plants I have found are:

• Stinkhorn (new models from the data addon) - T_Cyr_Flora_Stinkhorn01 and T_Cyr_Flora_Stinkhorn02.
• Mandrake - T_Cyr_Flora_Mandrake01.