2 to 3 year old Stans ZTR rim, Stans sealant used. I like to use normal tyres, hence the sealant.
OP, if you're blaming the sealant, please explain how it has got to the other side of the rim tape to rot the rim, yet the air inside the tyre can't escape?
Red herring comment? The sealant goes between the outside world and the inside air - the valve being a notorious fun spot when trying to seal these things, hence the various posts about the use of pliers. All you have here is a little rubber bung; some fluid always escapes.
I'm going to put this one down to a manufacturing defect and move on. I've never had this happen with any other rim and I've been tubeless for over a decade, hence why I'm slightly miffed. As stated, if it had been down to some trailside mishap, it would have been part of the experience that is mountain biking, and I'd have been happy. But I look after my bikes, so for this thing to turn to salt whilst sitting in the Man Cave is just not cricket.