I don't have Photoshop, nor have I ever used it. But just thinking to myself, I'm wondering if it could have something to do with RGB v/s RYB.

For example, in RGB, equal amounts results in White. But in RYB, equal amounts result in Black.

Maya operates in RGB as far as I know. So perhaps you are using RYB in Photoshop and it's messing up when importing into Maya which I don't think understands RYB.