I'm no expert on brakes, but have heard of 'anti squeal shims.'
From a quick search on line I have found various brands of self adhesive anti squeal shims .
Could it be that however flimsy and insignificant the bonded backing plate may have appeared , the manufacturers of that brand of pad found it necessary to go to the trouble of adding it to reduce the chances of squealing. (Whereas other some brands might have slightly different lining material, or a fraction of a millieter thicker backing plate,better machining etc and found shims unnecessary )
https://www.ebay.co.uk/itm/227230281982?Here is an example from Ebay just to illustrate my point. I am NOT saying this particular listing fits your car, or that its suitable,necessary or would solve your problem.