You would need to dispose the context. In WPF the bitmap is invalidated when its unlocked. In WBX the this is done in the dispose call. Use it like this:
wbmp = BitmapFactory.New(300, 300);
IMG1.Source = wbmp;
using(BitmapContext bmpc = wbmp.GetBitmapContext())
{
// Clear the WriteableBitmap with white color
wbmp.Clear(Colors.Red);
wbmp.DrawLine(0, 0, 1000, 1000, Colors.Black);
}