I assume open vented: The hot water flow circuit needs a valve to restrict flow through it. When the hot water circuit comes on and there is no restriction, the water simply goes the easiest way - 80% through the hot water circuit leaving 20% for all rads.
The other option is pieces of sludge or debris in the pipes moving around under different flow conditions. If your water is black - power flush the whole system. Draining it down will not be enough.
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