I have the same issue, connected those two loose wires under the seat of my BX2200 thinking it would help resolve issue I had getting power to the new turn signals I installed it didn’t work and instead caused this start and run issue, where engine stops when key is released.I had this very thing happen to my BX22. The issue I found was under the seat and no clue how it happened.
Under the seat are two wires, one with a female and the other with a male connector, that are for a rear light. Some how they made contact with one another and it caused the exact thing you are describing.
No clue if those wires are under the seat of a BX2200. If not, start chasing with wiring looking for shorts.
Also, make sure the fuel pump under the is functioning.
How did you resolve your problem?
Also, do you know what those two loose wires under seat are for?? lol