High efficiency furnaces belong to “condensing” type appliances (they produce a lot of condensation during operation) and lower temperature on exterior does increase exhaust gasses condensation, but this is completely normal.
The drawing below is probably something similar to your installation (except for dimensions which are important as well and may differ between models and manufacturers). If your PVC vent pipes are inline, one directly above the other, so the condensate is dripping onto the intake, you'd have to have them repositioned to prevent that from happening and to comply with installation manual and your local code requirements (BTW, exhaust above the intake is correct).
Condensation should be flowing from the exhaust PVC pipe back towards the furnace, drain through the system of condensate drainage pipes / tubes installed inside the furnace, and discharging into a floor drain or other approved location.
If you have those vent pipes exposed, examine each of the PVC joints while running your furnace for a few minutes. It’s very possible that one of them wasn’t sealed properly and is leaking. If this is the case, PVC pipe would have to be cut and the connector replaced / you cannot reseal such connection adding more PVC cement on the surface or using silicone or some other sealant.
Unless you can see the condensate dripping from the exhaust pipe onto the intake PVC pipe on the exterior and somehow penetrating house wall, leaking seal on one of the PVC vent pipe joints is most likely an explanation of your problem. Please let me know.