this may apply to Cruel enchantments also, but I haven't checked as yet. Other enchantments appear to work correctly.
I tested this with various belts and rings and +4/+5 enchantments on the Ioun Stone of Allure, Green Slime and Man at Arms companions with the same results.
while we're on the subject of these enchantments: their stats also don't show up in item comparisons (unlike other enchantments).
edit: I am 90% certain that all the problems listed above are merely cosmetic - as in, the enchantment values are getting added correctly, they just don't show up correctly/consistently on the numeric breakdown screens.
Comments
http://imgur.com/G1i0Nkk
http://imgur.com/3AyD2Pf
note that +65 Critical chance shown in the second image is from the Runestone, not the ring (which in any case is +50)
edit: I am 90% certain that all the problems listed above are merely cosmetic - as in, the enchantment values are getting added correctly, they just don't show up correctly/consistently on the numeric breakdown screens.