It’s interesting that you say the biscuit rises in the centre immediately it leaves the oven band. If a top/ bottom imbalance in the first 2 zones was the cause then it would be seen while the biscuits were still on the band.
So the fact that it happens when they transfer to the cooling bands would suggest to me that there is too much heat retained in the band at the end of baking.
If you can test by removing a few biscuits from the band and placing onto a cold surface and they remain flat while cooling then it would confirm that the band is too hot. If that is the case I’d suggest switching off all bottom burners in the final two zones. You will obviously need to compensate temperatures earlier in the oven and this may require you slow the bake down to 5 minutes or more.