Yes the damage to the plate is from going in reverse with the teeth down or lifting and rolling with some thing very solid hitting the grapple portion and not the bucket portion, that causes extremely large amount of force, and yes enough to just tear the steel at the weakest point.
Most equipment is not made to a standard of being indestructible, and in most cases it's just under built to save on weight and lift capacity.
You would be better off just getting it back, buy the thicker back plate, take it to a welding shop and have it done up right.
You will probably end up breaking the grapple at the next weak point unless you back off on what you are trying to do with it.
You have meet the limits of the equipment you are using.
I'm sorry but I just stating facts and I didn't write the warranty; but they have an out as the warranty says they are not liable due to negligence, or improper use, which they will say that it states in the manual not to overdue the limits of the equipment.