No encumbrance at all from heavy armor

I put on some Masterwork Iron Plate Mail (heavy armor, shown in the screenshot) which is supposed to raise encumbrance by 2 levels, but encumbrance does not seem to have increased at all.

I have also seen the same issue with chain mail (medium armor) that was supposed to increase encumbrance by 1 level.

That said, I have also seen the encumbrance effect work correctly for some armors. So, encumbrance behaviour only seems to be bugged from some of them.