Locks are sometimes drawn wrong

[Ters]

This has been a minor glitch that I have seen for some time, and I have reported it once before (http://forum.simutrans.com/index.php?topic=7672.0), but that was along with other glitches, some of which got fixed and the thread was archived. I have also noticed that the bug is not always triggered, which seems to invalidate the answer initially given.
Attached is a screenshot of two locks. The surrounding geography is very similar, yet one lock is clipped, and the other isn't. Why?

[Dwachs]

Hard to fix. It is completely independent of front / backimages. The error occurs if there are vertical walls behind the lock. Setting 'draw_as_ding=1' for the canal object only fixes it to some extend.

[Ters]

So it isn't just affected by the tiles immediately next to the lock, but also the tiles next to them again? It explains why this error is more common in urban areas.