Thanks for the reply.
While I did discover this while making a mod, I replicated the glitch in vanilla civ also. After I noticed that the RangedModDefense value wasn't being displayed in the combat popup, I set up an unmodded test scenario. Americans and Russians.
On the Russian side I created 3 warriors on open terrain - one with shock_1 cover_1 and the other with drill_1 cover_1 cover_2. The last I gave no promotions.
On the American side I created a crossbowman, which I gave accuracy_1 and a trebuchet which I gave barrage_1.
Then I disabled all mods and loaded up the scenario. I can see the -10% terrain penalty for defending on open terrain and the +20% bonus for accuracy. And you can see the targeted defender's Strength Value in the combat popup change depending on whether they have cover_1, cover_2 or no promotions. But the actual % bonus that's affecting that change isn't displayed with the other modifiers.
It seems strange that every % modifier except one would be displayed in the combat popup. That's why I posted this to the bug forum.