I have a one month old MG6150 and have this problem too.
I get around it by sizing the output to A5 but setting the printer to borderless A4...depending on your software you can set distances from the top and side so it lines up...might take a few goes to get it right but it is a solution...
I've been in touch with Canon over this as I reckon if you can set a custom size you should be able to print it whatever way you like...as for instance with HP.
Unfortunately Canon do not agree with me and their best Customer Service Advice is to discuss a refund with my retailer...how poor is that?
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