as the title suggests, rate of fire changes from attachaments do not show in the stats menu. they work in-game though.
a couple modded magazines also don't show the ammo changes, but these also work just fine in-game.

i looked in the code and the modded mags that weren't showing their mag size changes were using <custom_stats ammo_offset="x"/> (x having a value of course).
i have more weapon stats and weaponlib if that helps narrow it down at all.