TL;DR: Judging from the bars in tooltips, status resistance UVs are less effective on equipment that naturally resists that status than on equipment that doesn't. I suppose that could be intentional, but it sure looks like a bug, both because that's weird and because there doesn't seem to be a consistent pattern.

Long Version:

Based on my success measuring defense bars, I decided to try the same thing with resistance bars. I compared my total resistance wearing a Volcanic Demo Helm to my resist wearing the helm plus a 5* trinket, and found the bar was 20 pixels longer. Then I tried just the trinket, and found that if you started counting with the first pixel outside the circular icon indicating the type of resistance, the bar was 20 pixels long both in its individual tooltip and in the gear summary. So I concluded:

• 5* trinket = 20px resistance
• Bar starts with first pixel outside the circular icon

Armed with this information, I measured the bars for various armors:

• Wolver (2*) = 10px resistance
• Dusker (3*) = 20px resistance
• Ash Tail (4*) = 30px resistance
• Skolver (5*) = 40px resistance

A bunch of round numbers in an even progression! Seems like I'm on the right track. Then I looked at some resistance bars from armor UVs:

• Low UV = 10px resistance
• Medium UV = 20px resistance
• Maximum UV = 40px resistance

I didn't find a "high" UV, but I'm pretty sure I can guess how much it gives.

This is all just what I expected...until I started looking at the combined bars for armors that have a UV and have natural resistance to the same status effect:

• 2* Armor (10px) + Low UV (10px) = 19px
• 5* Armor (40px) + Low UV (10px) = 46px
• 5* Armor (40px) + Medium UV (20px) = 50px
• 3* Armor (20px) + High UV (est. 30px) = 44px
• Snarb Armor (30px) + High UV (est. 30px) = 51px

Um. I'm pretty sure this can't be a generic rule of diminishing returns, because the trinket added 20px whether it was by itself or on top of something else: this is something specific to adding up two bonuses that both come from the same item. And I don't see any consistent pattern; for example, the 5* Armor + Low and the 3* Armor + High should both presumably have added up to 50px, but one adds to 46px and one to 44px. The amount of missing resistance isn't proportional to the armor, the UV, their sum, or their product.

So, what's going on? This is a bug, right?