A modified diagram of the LM coolant loops:

The secondary coolant loop contains only cold plates for the electronics but no cabin heat exchanger. In the second loop heat is transfered from the cold plates only to the sublimators but not to the cabin. See the red arrows.

In the primary loop some cold plates are placed before the cabin heat exchanger and some after it. See the orange arrows. So only a part of the heat could be transfered from the cold plates to the cabin.

But both the primary and the second loops go to the same cold plates. So there is some heat exchange between both loops. If the coolant in the primary loop is warmer than that in the secondary loop, some heat flows to the secondary loop and is lost for cabin heat.

A modified diagram of the LM coolant loops:

The secondary coolant loop contains only cold plates for the electronics but no cabin heat exchanger. In the second loop heat is transfered from the cold plates only to the sublimators but not to the cabin. See the red arrows.

In the primary loop some cold plates are placed before the cabin heat exchanger and some after it. See the orange arrows. So only a part of the heat could be transfered from the cold plates to the cabin.

But both the primary and the second loops go to the same cold plates. So there is some heat exchange between both loops. If the coolant in the primary loop is warmer than that in the secondary loop, some heat flows to the secondary loop and is lost for cabin heat.

There is a Cabin Temp Control Valve, a bypass valve for the cabin heat exchanger. If it is too hot in the cabin, the valve may be closed to increase the flow through the heat exchanger. But if it is too cold in the cabin, the valve may be opened only to reduce the flow through the heat exchanger. If the coolant is colder than the desired cabin temperature you may only open the valve.