I'm not sure I would call it deceit, God is all-knowing. I would consider it taking God's mercy for granted and treating him like a mechanical forgiveness dispenser.

I'm hesitant to say it's always a mortal sin, because it seems like it can happen in a lighthearted manner. For example if someone struggles with gluttony, that person might think "I know I shouldn't have a 2nd hamburger, but I'm weak and it would taste so good, I'll have it now and go to confession later". That doesn't seem like mortal sin territory to me.