So your chain is rubbing that stay/dropout all the while it's in the 11 cog? Or does something else have to happen like the chain comes off or something. I too have to wonder if the high limit isn't set correctly.

I assume you took it back to the shop where you got it since you got a statement from Giant on the warranty aspect. I might just take it to another shop and see what they say.

IMO, if the chain rubs the stay/dropout all the time when in the 11 cog and everything is assembled and adjusted correctly, then that might well be a warranty issue and mfr defect. However if it's just someone put it together wrong at the shop or didn't adjust it correctly, then that's on the shop or whomever futched with it.