Mains supply to mixer tap is pushing cold water to hot water pipe

I live in an apartment block where generally hot water is supplied from the communal hot water cylinder and cold water to bathrooms is supplied from cold water tanks in the loft. It is a vented system. However, in a basement flat and a ground floor flat cold water supply to bathrooms is through the mains (as in the kitchen of course). Because the pressure from the mains is much higher than for the hot water supply, in some mixer tap the mains water in the mixer tap pushes mains water through the hot water supply into the communal hot water cylinder and then hot water rises into the cold water tank in the loft creating an overflow....

What issues should we be looking at the mixer taps for this problem? We have not identified the mixer tap yet.

Many thanks,

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