This is very strange. Forget towns and cities, model textures have always been a cause of poor pathfinding, units getting stuck, etc. I am curious about the problems occuring on maps like Grassy Plain. The drag and drop feature was a new introduction to TW when RTW was released, it caused a few problems for those familiar with the old alt/ left or right click commands. I tried many times to replicate the issues that some people were posting but my armies always carried out the commands correctly, even when using multiple groups and alt/left click.
I mentioned play testing my historical battle earlier and first noticed the change of some unit deployment while doing that. At first I thought it was one of the problems I'd read about but I was wrong. The drag and drop command allows you to easily redirect your army, for whatever reason, by simply scrolling out a new destination; I did this and found my original deployment pattern was restored. When I looked into this a little closer I found that my army had altered its formation due to terrain and its current formation showed up with drag and drop, however when it arrived its original deployment was maintained.
I've never witnessed a problem like this on Grassy Plain or any other fairly standard terrain maps (after many MP battles) so please don't take offense when I ask you exactly how do you issue commands?

My method....

Deploy army
Group army
Select group
Drag and drop

Division of army into into sub groups can be done near to engagement.

Another method but slightly more work....

Deploy army
Sub group certain units
Select all
Alt/left click to location
Use < and > change facing

Both methods see my army line up in original deployment at their new destination point unless there is an obstacle

........Orda