Interesting ... as I am not too familiar with UTVs, which is part of my problem in trying to fix anything with it, and I didn't know they had a spark plug cap (unless I'm thinking of a distributor cap) and wasn't even thinking of the coil. I have the same year and model as you (2021 UTV 400) and have found limited information on doing repairs.
I'm old school and grew up working on vehicles with spark plugs, spark plug wires, distributor cap, points, rotors, carburetor, etc. Now with my 2019 truck, whereas everything under the hood is electronic and covered with plastic covers, I'm totally lost.
Looking at the UTV, for me, it's a totally new animal and I can't figure which way is up with this thing, but I'm relearning as I go along.
Going back to old school, carbon fouling is usually a symptom of a fuel problem. If you have verified no water in the gas, then it could be fuel/air mixture, but you should have injectors, and the fuel/air mixture I believe is controlled by the ECU and you reset that. Try checking your air filter to make sure it's clean and installed properly. I remember reading some place that these rigs came out of the factory, in many instances, with the air filters installed backwards, which had caused some problems. Also check your idle speed to make sure it's not idling too low, although I think it would take a while to foul the plug with a low idle.
If I think of anything else I will surely forward it to you ...