RFC 1738 and 2396 both say schemes are lowercase but implementers "should" treat uppercase as equivalent.
RFC 3986 explicitly says schemes can have uppercase letters but are canonically lowercase, and says that they are case-insensitive. But it also still says that implementations "should" accept uppercase letters as equivalent.
WHATWG's URL standard mandates case-insensitive matching.
In practice anything you scan a QR code with will probably not have a problem with the uppercase scheme.
RFC 3986 explicitly says schemes can have uppercase letters but are canonically lowercase, and says that they are case-insensitive. But it also still says that implementations "should" accept uppercase letters as equivalent.
WHATWG's URL standard mandates case-insensitive matching.
In practice anything you scan a QR code with will probably not have a problem with the uppercase scheme.