I’ve managed to find this problem in the issue tracker, but the ticket UE-15446 (unfortunately looking dead for over a year) seems to describe it as a feature request, while it clearly looks like a bug, because the current behaviour doesn’t seem useful in any capacity. The issue talks about Vertical Box, but the problem is exactly the same for Horizontal.
Just to reiterate on the steps to reproduce:
- Put an image in the Horizontal Box, with the image height bigger than box height.
- Wrap the image with a Scale Box.
As a result, the image reports properly shrunk width, but the Scale Box has width equal to unscaled image. This makes using those automatic layouts together with Scale Boxes practically useless.