Steps to reproduce
Unfortunately, I wasn't able to detect the exact use case which is affecting a customer upgrade, however I'm sure that the issue's root cause is the fact that old versions of a Web Content Structure are overwriting the latest version on it. The issue was observed during the upgrade of 7.1 to 7.4, but it's probably reproducible in a upgrade from 7.2 to 7.4.
Some Web Content Structures are upgraded with missing fields
All Web Content Structures are upgraded with all their original fields