Mixing valve broken?

[taanio]

Moved in recently and inherited a combi boiler with 2 zones setup, one to the rads all around the house and a second to the underfloor heating in the kitchen. When the kitchen zone is activated, the boiler kicks in, the flow pipe heats as expected and the heat only travels through the pipework to the kitchen zone. It then gets to the mixer valve for the underfloor heating, where I would expect the heated water to travel through and down into the underfloor heating. This does appear to happen as the pipe below the valve starts to heat up straight away, but the cold side of the mixer valve also heats up instantly. This means the hot water gets part pushed through the underfloor, but also (and I think the majority of it) gets pushed straight back to the return on the boiler. The boiler sees the return get up to temp after about 30 seconds and shuts off.

Am I wrong in expecting that the mixer valve shouldn't be doing this, and that flow across the mixer valve should only be from the underfloor return to be recirculated?

Thanks

Martyn