Hmm, how far off idle were you when it stalled?. Was the bike warmed up before riding off?.
If the dealer is willing to come that distance at no cost to you, then let them. Discuss how you feel calmly and see what they will do for you. My feeling is that the biggest problem with this bike (probably most Enfields) is QA at the factory. A lot seem to come out, out of "spec" as it were and need some fettling to get right. That can be a number of things, valve clearances, plug changes, checking hoses, loose connectors etc. This is stuff one could argue the dealer should check as part of prep. But if they did the prep cost would be exorbitant. Yeah, RE need to lift there game here definitely. I almost got an old carbed version due to concerns re the EFI version. But if wanted an old school carbed bike I would have got a DR650.
Mine has very few Km's on it and likes to stall when warming up, got to the keep revs up . Idle is very lumpy until warm. It reminds me very much of a carbed bike. I read that these run quite lean down low. Plus they also have a fairly rudimentary EFI system.
Try an Iridium plug and see how it goes?. I going to try one on mine to see if it idles better cold.