It does seem silly but if you aren't used to DSG you will sh*t your pants all the time.
Going down a hill it'll randomly down shift for the engine braking,
Uphill it won't change gear and it will rev quite high,
If you're in S it won't change till 5/6k
If you tap the paddles it temporarily keeps it in Manual mode fr 30 seconds, I often think whys it not changing, and then i realised 200 meters ago i upshifted with a paddle.
So yeah as a summary it can be difficult to understand, however I would strongly say its probably not the Mech unit gone.