The overlay map toggles automaticaly off after each zone change and after opening any sub-menu like inventory or passives and such. Trading seems fine though if you close the windows manually and not via esc-key (which toggles the overlay map off as well).
I like to play with the overlay map toggled on all times so this is a bit irritating.
If this works as intended see this post as a suggestion.
Alpha Version 0.6.1.4