If you look on the inside of the big chainring, you will notice an number of pins and ramps that pick up the chain and lift it onto the larger chainring - the FD just pushes the chain sideways to where it makes contact - the pins and ramps do th actual lifting. These are positioned to pick up the chain from the assumed matching inner ring. If the small and large rings are not matched (50/34, 52/36, 53/39), then the pins and ramps will not be in the right place. This doesn’t mean they become the Chainrings of Death, but shifting quality will suffer.
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