Could you please state your vendor ?
The degradation could be due to the fact that all the dcs1800 resources are occupied (which is a good thing). Because all traffic is carried in this inner zone, you understand that you fully use the benefit of the DCS1800 band.
However, the BSC will still try to push more traffic in the inner zone, which leads to incoming HO congestion : HO from outer zone or from neighbor cells towards the inner zone cannot be done because no resources are available.
This is not a problem 🙂 You have two possibilities :
1/ share the traffic equally between GSM900 zone and DCS1800 zone. Bad option in my opinion, because you should load the inner zone as much as possible.
2/ stop watching the (HO + normal assignment) congestion rate, but watch only the (normal assignment) congestion rate. You will have a much better view of what is the real QoS in this cell.