When pressing tab to open the map, all I see is the marker for where I am and an arrow for my friends position, but the actual map isn't seen. There are no lines or anything, just a dot for where I am. This happened on pumpkin pastures, for the entire level. The person I played with could see the map however on his computer

(the picture below is from a different level when the issue happened again). 

I attempted to reproduce the issue with my friend. It happened when I was alone (see below) and when i did it with him. However, during the 2nd and 3rd time it occurred, part of the map was loaded fine, and as we progressed, the map just stopped loading, so there would be a half complete map.