I have. I set off a short way to run an errand during very cold weather and dropped anchorwoman off for a short while. The heater actually went colder to the point I had to turn it off as I was freezing. When up to temperature, the heater actually gets very hot but it isn’t a heater issue it’s something that all modern diesels suffer from. A diesel engine is very “thermally efficient” and so most of the heat it produces is turned into energy and sent out of the exhaust. As a result, the more efficient they become, the longer they take to warm up. Petrols have poor thermal efficiency and warm up quickly as the heat is soaked into the engine block. The diesel even has a 1kw electric heater in the heater intake to help take the chill off but still it struggles. There is nothing you can do except put up with it. Heated seats and steering wheel help.