I was not able to reproduce the same situation with demo controls.
But i found what the problem is. This happens when column has relative value for Width property, like this:
When column's Width is fixed value than everything forks fine.
We are using such values to adjust grid columns to strech them through all available space for grid.
Another question is how not to show grid lines for not used space without columns, it looks like there is another column without header defined.
Is it possible to set something to avoid such behavior (see attachment)?