I had a very similar issue and solved it with Decode function. So in your case, you would do something like this:
Decode($feature["fire_fuel_treatment"],
"biomass", ["chipping", "lop and scatter", "masticate", ""],
"experimental", ["assisted migration", "inoculation reduction", "test new vegetation management tools", ""],
, "")
Not 100% sure if it works in Field maps, as I used this script in a different app, but I don't see a reason why it wouldn't.