I've never heard of the issue. I have experienced breaking down of the waterproof coating due to moisture. I suppose if you leave a tent on the plastic sheet for a long time it could start to break down the coatings if it traps moisture. In my experience the breaking down of the coating takes years not days.
I no longer use a footprint backpacking and haven't had any issues. People who do use them recommend tyvek or polycryo.
Thanks for your suggestions. I guess mould could harm a coated ground-sheet.