Sometimes the character hands are drawn on top of the map, blocking view of the map corners, as opposed to under the map which typically is the case. See attached screenshots.
I don't have specific repro steps unfortunately, but this seems to be frequently happening while using boats.
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Still happens in V1.0.9
To duplicate the effect:
If you enter the boat with a map in the ready line, if you view it without doing anything first the hands will be under the map. On the other hand, if in the boat you open and close inventory and then look at the map the hands will be on top. To get back, you need to exit the boat, open inventory and swap the map out and back in to ready, close inventory, and reenter the boat.
I seem to remember that the same effects happen if riding an animal.