Brand new XT1 54" Fab trowing the deck belt

Hey guys. I purchased a new XT1 54" fabricated deck after 16 years of a JD L110. I had it cutting 30 minutes before the deck belt threw off. I was cutting in some non-prime conditions, as some 1" sticks were around. I put the belt back on and picked up sticks before cutting the rest of the yard. The thing cuts great, steers easily and is very fast. I cut grass two more times. My brother in law was over and I was bragging on the beast and I wanted to show it off. I cranked it up, had it at idle and hit the electric PTO. The dang belt threw off again! The angle from the engine pulley to the deck pulley is radical at a 2.5" cut height. I was so pissed I took it back to TSC for a refund and bought a 54" JD D170. The JD is way slower and steers harder, cut is terrible. Is anyone else having the deck belt issue on these 54" fab decks? I will try another one if no one else has had this problem but I think it would be unlikely that I got one with a bad deck or something. A quick search here revealed the same issue with a 50" machine. The manual does say to start the PTO at full throttle, but that goes against my home training. Thanks.