Exploring the missing parts
Here you go, just look at each map and see that all the named sections are uncovered.
I’d advice to ignore it instead of tearing your hair off.
I got 100% exploration with 2 characters and still have 172/173 and similar counts for several of the zones. Even when trying to complete it I found like half a dozen new areas yet none counted towards completion for some reason, even those areas other people suggested as the missing ones, nope I get the “you have discovered this area!” notification but nothing changes.
Map completion is only determined by hearts, waypoints, vistas, skill points, and points of interest. There’s some areas that don’t have any of those, so they’re easy to oversee. I had to check out the completed maps on the wiki to find out what I was missing. It’ll be named areas so they shouldn’t be overly hard to find.