The AW3423DW has a GSync Ultimate module, so it can't have a built in scaler which the AW3418DW has since it's only GSync (not Ultimate). This is to reduce as much latency as possible by not having a scaler to pass a picture through. The scaler would explain why you were playing as you describe previously.
You will have the same issue if you were to buy any other GSync Ultimate monitor. I don't know if FreeSYnc Premium Pro monitors also have a scaler, but since they aim for the same spec/performance targets, I suspect they will follow a similar criteria.