The issue is that QR's alphanumeric segments are uppercase only, and while browsers will automatically lowercase the protocol and domain name, you'll have to either have all your paths be uppercase or automatically lowercase paths. On top of that when someone scans the code it will likely be presented with an uppercase URL (if it doesn't automatically open in a browser) and that should alert anyone that doesn't already know that uppercase domains are equivalent to lowercase domains.
Ideally QR codes would have had a segment to encode URIs more efficiently (73-82 characters depending on how the implementation decided to handle the "unreserved marks"), but that ship has long sailed.
A significant downside to the <picture> element, and alternative image formats in general, is that when most users wanna download the image they expect an image format they already know how to work with. To most users an .avif or .webp are an annoyance because they reasonable expect most of their tools to be unable to open these.
It's disappointing that browser vendors haven't picked up on this and offered a "Save as PNG/JPEG/GIF" option when downloading images, but for now if it seems reasonable that if any user would want to download an image you're displaying then you should probably stick to the legacy formats.
Ideally QR codes would have had a segment to encode URIs more efficiently (73-82 characters depending on how the implementation decided to handle the "unreserved marks"), but that ship has long sailed.