3G to 2G per Neighbours stats shows HO attempts were made to undefined 2G Neighbours from 3G cell. How it is happening?
This is probably a db mismatch. Ideally, for a handover to 2G, you are defining not just the ARFCN but the cell ID and other details as well and hence a handover to "unknown" cell cannot happen.
Vendor's reply is that 2G neighbours may be defined for any one cell in Active set