Hello Luba,
At first, as I stated in my previous comment, the problem was solved using another basemap. After reloading the map, however, the problem reoccured. I've tried the same map on a different device (Nokia) and the problem seems to be gone again. It's not a very consistent problem it seems. I'm hoping the problem was caused by the device and it won't occur for the field workers.
For now, thanks for your time and help.