C5 X7 3.0hdi sudden coolant temp drop

[Paproch]

Hello

I have a 2011 3.0 HDI equipped with a Webasto system. There's a problem with the cooling system.
The car warms up normally, there's no overheating.
The problem occurs with the water temperature reading and the interior heating.
When the car is idling, it warms up slowly and warm air comes out of the vents.
When driving at 2000 rpm or higher, the coolant temperature stays around 80-84 degrees and the vents heats. No matter if 2000 rps are at 60km/ or 125km/h.
When maintaining an RPM of 1200-1900, the temperature reading is correct, but there's no interior heating (no mater what is set on clima control)
If I'm driving and the temperature gauge shows 6-7 bars and I suddenly brake (e.g., when the lights are red on crossway), the reading immediately drops to zero bars, and the FAP app shows a temperature of around 45 degrees. At that point, there's also no warm air.
The coolant, thermostat, and coolant temperature sensor have been replaced. The heater matrix is clear and there's no debris in the system.
Where to look for a solution?

Have a nice day

[wurlycorner]

That's definitely odd sounding behaviour.
My only thoughts are that there is air in the system and/or that the control system of the webasto is somehow faulty.
Was the system bled correctly as per the correct procedure in the service manual (I assume there is a different procedure for webasto cars compared to non-webasto cars?).
Was it a vacuum fill or a traditional fill?

[Paproch]

I couldn't find any instructions for webasto cars. So it was made traditional way, with normal filling, non-vacuum.
About webasto- i think there is no such a amount of coolant to be able to go inside cooling system and drop temperature for so long.
Drop of temperature is for 2-3min, not seconds
There is any additional coolant pump or something like that? Becouse when I stop the car, temp drops, and while idling I'll rev car to 2000rpm for let's say 15seconds, temperature is rising to 75+ degrees. But when I stop putting throttle and revs drops to nominal 750rpm, temp also drops.
So it's somehow connected with fluid speed in system

All that started around October, when outside temperature started to be around 10 degrees and lower. So it also can be connected with webasto :/

[wurlycorner]

This is beyond my experience with the specific car, I'm afraid. I think this needs someone who is familiar with the Webasto set up - I would expect there is an auxiliary pump for it, but I don't know.