How to tell why a patch was aborted for a full update?

Our English release seems to update properly.

but our Nipponese release seems to always believe that it needs to abort the patch, and use the "full" installer.

Is there a way to see what file failed the test and caused the abort?

Yep, you can tell exactly which file is failing: https://wyday.com/wybuild/help/faq.php#catch-all

I'm thinking we need a way to tell without having to rebuild anything / re-distro anything. A log file or included in the error message or a windows event.

An aborted update should only happen if (a) the customer messes with files (in which case they should not do that or accept that the update files will be larger) or (b) you've made a mistake in your deployment or patch management, in which case you can fix it using the FAQ method.

We don't like spitting out logs on systems because 99.999% a customer won't need that information.