I’ve had two XM819 rims fail – both in the same way – the internal weld that runs around the “bed” of the rim (where a rim tape would normally go) cracked.
First sign of the problem in both cases was a gradual loss of pressure; close inspection then revealed a fine jagged crack starting from either side of the valve hole. In the first case this happened at a rather awkward time (I was cycling around Tajikistan – about as far away as you can possibly get from a bike repair shop); I fitted a tube and rim tape and managed to “nurse” the wheel for the remainder of the trip (about another 500km) – by the end of the trip the crack went almost the whole way around the rim.
Suggestion from Mavic was that I’d fitted too wide a tyre and/or inflated the tyre to too high a pressure – neither of which had happened. They also suggested a poorly built wheel – not the case – it had been professionally built onto Chris King hubs. I thought it might therefore have been a one-off, but exactly the same problem happened again a couple of weeks ago. I’ve now ditched tubeless rims and gone back to Mavic EN521.