How to Snub Voltage Spike Created by Appliance?
If it's a spike, a surge suppressor on the dehumidifier should fix it. Surge suppressors are non-directional.

If it's a drop, that won't help. A drop could be from a poor connection with high resistance that is more noticeable when the large load of the dehumidifier starting is applied, or it could just be that the circuit is heavily loaded.