Same thing happening to my 2014 Seat Leon FR 184. It's very frustrating.
I had my friend, who is a mechanic look at it and he went the same route as a lot of the members here and diagnosed it as a blocked heater matrix but could have been a faulty water pump. We replaced the water pump and flushed the system a few times. The heating came back but the persistent problem of the coolant fluid being pushed out the tank still continued.
I read up and narrowed it down to it happening when the DPF regen kicked in. It was interesting to read the different types of regen that can happen but what I found interesting on one of the sites I read was that the regen kicked in within 50km (or something like that) after you filled your car with Diesel. Seems to be happen everytime for me. So I've monitored when the engine revs get uplifted by the ecu and sure enough I lose coolant.
Reading this thread has been fantastic and so I believe I am left with 2 options to resolve it.
1 - In Ireland (so I read) we can legally remove the DPF, the sensor and stop it on the ECU (there is a place in Dublin that specializes in it). Cost €550. Result, the DPF does not kick in and the load does not come onto the heating matrix.
2 - Replace the heater matrix to the improved one you guys are suggested in the car myself and flush the system to make sure everything is clean. Cost €100 plus my time.
Needless to say I am following the advice previously in this thread and going with replacing the heating matrix. I see a Nissen one and a Valeo one - I assume both work fine so I am going to order one up and give it a go.
On another note I have to agree that it is worrying that the VAG range of manufacturers do not know what needs to be gone and how to diagnose this properly. I purposely did not go to a main dealer to get it looked at because I think that they either a) know the problem but know they can charge you for further work; b) it's a known issue/defect and they do not want to admit it and get recalls or c) they simply do not have the experience to diagnose things properly ...... I am talking about this from past experiences.....but enough of my conspiracy theories
I'm just delighted that groups of people on these forums can come together to troubleshoot these things to a point where we have confidence in the solution.
Keep the post coming and I'll let you know how mine goes once I get it.