Lost my RHS cover at 100kph (60MPH), never found it as it disappeared over the edge into the bush, but I got it and the rubber grommets replaced under warrantee.
I would try the dealership and have it replaced under warranty. Had a 2005 bonneville and the side cover flew off, The dealership replaced it and grommets. How about the earthquake last night?
I have to be right careful putting the covers on, to make sure they engage the grommets. If they don't grip properly they do come off easily! But Triumphs are not unique in this...my Suzuki Intruder covers go on the same way. And I have even put new grommets on it for the old ones were well past it.
Been discussed here before -- you could do a search -- helpful function.
@cr0ft showed us how to drill a hole in one of the posts, and double-wire-tie the cover to the frame. At least that way, you'll have some notice of the cover separating and not lose it.
I would have made him call while I was present. Motorcycle dealerships are like used car lots, if they work there and their mouth is moving guess what their lying. Hope it works out for you.