There seems to be a lot of bugs like this in Hugo. Another problem I ran into is functions returning weighted pages, when there is no discussion of what weighted pages are in the docs. Yet another problem is that accessing .Reverse reverses the actual list instead of returning a reversed copy of the original. The only way I could figure out what Hugo was doing was by reading the source.
The simplest solution was to stop using Hugo. Jekyll may be a lot slower, but it does what it says it does!