V40 low coolant issue

[Nabman]

I have a 2024 V40 1.6 Diesel. A few weeks ago we started getting a low coolant warning and then an overheating warning. Left the car with a garage who done a full head gasket pressure test, drained the system, refilled and cleared the faults. No issue with head gasket found. 5 miles later the warning came back on. Left the car back and this time they said the system was air locked and low on coolant. Replaced coolant etc and gave the car back. Same problem again, low coolant warning, high temp and now going into limp mode. Left the car back and this time they replaced the water pump and thermostat. Cleared all faults and on test drive the problem is still there, tested head hasket pressure again but test was negative.

Has anyone experienced this? When the car is cold, the coolant resevoir is sitting at normal levels but when it heats up the resevoir competely empties and the engine seems to retain the coolant. The level will not return to normal until the cap is released on the resevoit and the pressure gushes back to the resevoir. There doesnt seem to be any coolant loss that we can see. Also the heating in the car works intermittently.

I have been advised by the garage that the next step is to replace the egr cooler as it may be cracked. I dont want to give them the go ahead for this unless they are certain that it will solve the problem as it has cost me £750 already with the issue still not being solved. If anyone has any suggestions, please help!