Doesn't seem to be a known bug of any kind, so assuming you tracked this properly I'd be going with outlier level of bad luck.  

I run it pretty much every fight now, and it seems to be 50% as expected.

Running Stoneskin against buff strippers should be avoided anyway. Or run multiple. 

I see it more as a weighted coinflip. Serris is the worst of them, sure, because she's also running 500+ ACC. But Kymar, Kimi, etc - they buff strip, sure, but usually they're also fast, which means they rarely run higher than 400 ACC.

I run 500 RES on my Stoneskin Cardinal, so I have a 50% chance to just ignore the move completely, and the remaining 50% of the time, I figure I have at least a 75% chance to resist. Sure I'll lose some, but if I'm against an opponent who is higher than me in points, I'll just fight them again, and average ahead.

Maybe res is part of my problem I've only got 308 on my stoneskin nekerhet, but meh I don't have enough decent stoneskin gear to really up it at all.

I'm just gonna chalk it up to being really unlucky from the sounds of it anywas.

Yeah at 300 RES you're basically just relying on the normal coinflip. Even a Kymar/Kimi/etc will have no trouble removing that stoneskin if they don't get hit by the 50%.