Dip in subfloor during hardwood floor installation

I'm trying to put in 3/8 inch engineered hardwood flooring myself, and would like some help.
We have 3/4 inch OSB as our subfloor in our bedroom upstairs. On some advice, we started to nail 3/8 inch CDX plywood to add some more structure to the subfloor. We chose 3/8, so that we would be level with our bathroom flooring.
While nailing down the plywood, we noticed that the floor dips downward towards the entraceways to the room. Starting about a 1.5 feet before the entrance to the bathroom as well as the hallway, the floor starts to dip, reaching a depth of app. 3/16 of an inch at the doorway of the bathroom and hallway.
The hardwood, once installed, will run lengthwise into the dip.
How do you go about leveling the dip? Is it something we should be concerned about? Any advice will be greatly appreciated.
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