These numbers are confusing me. Sorry.
To test the plate first you need to take a hit of less than the plate protects do you not? So case in point - Impact protection. Using a 5B go for a Daikiba or Bery
Young (has to be a young) and take a hit.
Check the decay on both the base armour and the plate.
You will need to repeat this test as there is no one mob that delivers 100% impact at less than or equal value to the 5B or even 8A max impact protection. Unfortunately sweating chirpies makes them run off not attack.
Until you have done this a few times and see if the base armour decays
every single time you can then say the base armour decays at least the minimal decay even if the plate sucks up mose of the dmg.
Alternatively try Allophyls with Shogun and 5B. The Allo should never make over 9dmg in impact if the figures are correct on wiki (26 dmg * 0.33 (%) = 8.58 dmg)