It's the hoodrats that do this. They will always steal the MC vehicles once you start fighting the lost. Have one member hang outside the door and kill them as they "escape".
Don't tell R* this, but there's a simple fix:
overpowered Insurgent, attach a proximity mine to each door, and pick it up unscathed when you're finished.
Re: the BeeJay XL and its "ability to sustain heavy damage may be inspired from real-life Toyota Land Cruiser's (especially the original FJ) exceptional reliability, longevity, and toughness." (from GTA Wiki)
Try and blow this vehicle up via the gas tank method (shooting directly at the upper left area of the rear left side wheel well), and you'll have a long wait, even using a Combat MG...