The file below illustrates the problem.
Create a copper area on top and bottom of board. Create some vias in between. Mask the top and bottom solder mask (with a rectangle) as you would if you wanted bare copper to solder a mosfet to for thermal reasons. Render the photo or gerber. The bottom surface correctly renders. The top does not. The vias are not visible.
[https://drive.google.com/open?id=1c2K32OgADLJ835bzxWHK_RrQzdrTMS1Q](https://drive.google.com/open?id=1c2K32OgADLJ835bzxWHK_RrQzdrTMS1Q)
Chrome
67.0.3396.99
Windows
10
EasyEDA
5.6.10