Either the regulator or the wiring integrity is at fault. But, your alternator appears healthy.
If you still have the stock regulator, almost certainly the adjuster was tweaked by the clueless. At that overcharge rate, the electrolyte will go into electrolysis (separating into oxygen and hydrogen, looks like boiling.) Those gasses are highly flammable and explode in confined spaces, such as inside the battery.
The regulator on this bike is a charging limiter. It regulates the system indirectly by keeping the battery at full charge.
Spitting back can be too lean, faulty timing, incorrect valve clearance, a burned or bent valve, or poor vacuum sync.