Neither Malthor nor the Mist Dragon had a Spell Mantle in their buff sequence (Malthor has Improved Invisi, Mass Haste, and Epic Mage Armour). Moreover, Assay Resistance has "Spell Resistance: No" which to my understanding means it ought to ignore Spell Mantle (
https://nwn.fandom.com/wiki/Spell_mantle : "Mantles do not absorb all spells, just the ones that check spell resistance" ).
The log unfortunately showed nothing informative. There was the VFX from the Assay Resistance striking its target, but my spells were still failing very frequently (as opposed to when I hit Malthor with Disjunction, at which point most if not all of them work).
I'll try to see if I can isolate some logs next time I'm in the situation.