I have not seen that. Could you post a screenshot?
EDIT: I misinterpreted the term "grid mode". Here are two screenshots, one with Grid ON and the other other with Grid OFF - if this is what you mean then the answer is clearly shown.
The route runs fine with everything functioning as expected. Most of my rotes are built upon baseboards in a rectangular shape. When I go into the grid mode (F9)some of these baseboards shown the black grid lines and others just show a solid white surface. In both case all the assets are showing, except of course the texture.
i am not experiencing missing base boards or sections of the route in those areas. Is this a bug that was introduced when the darker rendering of the grid lines was introduced along the way to the final release of TR19?