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.
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.