For full context, please see these two threads on Community Central.
To summarize, in addition to pre-existing issues, the version of Module:Mbox provided to new wikis appears to now experience yet another problem. The default inline CSS added by Module:Mbox/data contains a mix of shorthand properties and longhand properties intended to override the shorthand properties. One such example is the general border width intended to be overridden for the left border.
Upon inspection of an ill-behaving template, it was found that the order of CSS properties in the generated HTML appears to be arbitrary. As such, the general border width can sometimes be set after the specific width for the left border. The impact is that some templates result in a wider left border and others do not.
As HumansCanWinElves already pointed out to me, this makes sense considering that there is no guaranteed order to table fields. However, this is the first time I have seen this problem. Prior to UCP (or maybe even for a short while after), none of the wikis I helped use the starter module ever had this issue. I even went back to check on one of them and it still works perfectly fine on that wiki.
Even if this has always been a risk, does anyone know of any incidences prior to UCP? If not, are there any ideas of why UCP (or perhaps some update since UCP) might cause this issue while the legacy platform did not?