Construct's Armory: 1.12.2-1.2.5.4
Tinkers' Construct: 1.12.2-2.13.0.183
Mantle: 1.12-1.3.3.55
Forge: 14.23.5.2847

I was messing around with mixing Construct's Armor Resistant modifier and Vanilla Armor Protection. As expected, protection 4 on a vanilla set of armor offers the same EPF as resistant 8 on a conarm set of armor (64% reduction, verified with fall damage).

I stumbled on this bug when I started mixing and matching the Protection pieces with resistant pieces. When you are wearing pieces from both armor sets, you don't get as much EPF protection as wearing either of the full sets.

10 minutes after I submit the report I realize that the protection doesn't stack on top of each other, it gets applied separately. Wearing half and half would mean that 32% is blocked, then an additional 32% of that is blocked, so 53.76% not 64%

Interestingly, this means that If you have 3 conarm max resistant pieces and a vanilla piece with protection 16-20, you have more protection than armor with maxed EPF. This also works if you have a total of 18-20 protection levels spread across 2 vanilla armor pieces, But it isn't as good as using only one vanilla piece.