As the title says, when I rotate a simple rectangle its height-width relations change.
This is really weird...
can you you show pictures of your problem?
Sure, but there's nothing much to look at -- the left is the original, the right is rotated:
The transform panel is your friend, see how the bounding box and dimensions are the same when the rotation is 0º
when the rectangle is rotated, the bounding box shows the correct dimensions
If your rectangle is a Shape, you can see the properties in the Transform panel.
The Bounding Box changes when rotated, but the rectangles width an height do not.
It's a simple rectangle made by the Rectangle Tool...
What does the Transform panel show you?
Okay, this is weird -- I can't reproduce the error now.