Since MP1 didn’t 3-bet the turn, perhaps he has a baby flush or a single spade. He’s almost sure to have a flush with the 4th spade on the river.

Given your Turn CR, he’s got to be thinking you filled up. But with 10+ BB in the pot, he’s going to pay you off.

BUT, there is a chance he just has trip Ts & was semi-bluffing on the Turn. That may be why he didn’t 3-bet the turn and was drawing for the boat. If that’s the case, he may fold to the River bet. If that was your line, I can understand the River bet.

SP