It might be similar to the fear of fire. After they are burnt, sims will have fear of fire and the fear goes away if they manage to extinguish a fire. I think if you click on the fear icon it will have some description of the fear and how to cure it. I found this on Google, not sure if this is something you tried.
Fear of Cowplant
If a Sim gets swallowed and then spat out, they develop an intense fear of Cowplants. To get over this fear, let the Sim successfully Milk a Cowplant, while being Confident.
Maybe they need to be extra confident for this to work.