Since when does SDSM bump spell failure by 45%?

[Adieu]

My Vampire Chaotic Wizard wished in SDSM (Silver Dragon Scale Mail) nice and early, put it on, got a major happy for all of 5 seconds... and then realized I couldn't reliably cast ANYTHING anymore. When did that happen?? All spoilers/resources/etc seem to claim that Dragon Scale doesn't jinx wizards' spell failure.

Tried without wearing anything else, with cloak, without weapons, etc. - same thing. Wasn't hungry, wasn't burdened, wasn't cursed, wasn't unlucky. All stats were high, including highlights like 18 int, 18 wis, and a whopping 24 str.  

Short of a version change no one warned about, I cannot seem to find any other reason for this issue.

What gives?