I'm playing a monk with Diamond Body feat and am immune to all poisons.
But recently, when I get hit by Darklings I seem to get poison reduction to my STR anyway.
I get a message saying "immune to poison", no fortitude roll, but the stats are reduced anyway.
Unless it is something else the Darklings have in their hit damage that reduces STR? Something with out any saving throws...
I haven't tried fighting other enemies with poison as it would likely lead to my death very fast.
I took a screen shot but cant find it now, so you have to do with imagination.