Perhaps the OP used the multi-city function on expedia and entered sector by sector (LGA-YUL, YUL-LAS, LAS-YUL, YUL-LGA)? I just tried a November dummy booking for that through their website and a pricing came out but at $2,572.80 CDN which is way off the amount the OP is claiming. But it requires to overnight in YUL on both directions (less than 24hrs both times) since there are no LGA-YUL that will connect same day to YUL-LAS. Same goes on the return with LAS-YUL arriving late to not be able to catch a same day YUL-LGA.

But if only inputing LGA-LAS vv as the city pairing roundtrip, no way would an option via AC metal be an option. Must be some weird tech error.