The 'tunnel between the fridge section and the freezer section on a single compressor model allows the colder air to enter the fridge from the freezer and when the fridge is down to temperature, the flap shuts to stop any further cooling, while the freezer continues to cool.

If the flap across the tunnel becomes blocked by frost/ice, then the fridge section can over cool past the required temperature resulting in frozen milk etc.

If you have a single compressor fridge freezer and you are losing control of the temperature in the fridge section, then the problem is 99% of the time something to do with the tunnel flap.

The flap operates via a thermostat so if you have done a 48 hour defrost and still have problems, then a close inspection of the flap is required. If it is clear of obstructions, the next remedy is to replace the flap thermostat.