This engine configuration is notorious for coolant issues; look at the Briskoda Skoda forums too. Issues include:
Main water pump and associated seals.
Cracked welds on EGR cooler.
Blocked/corroded heater matrix.
Cracked expansion tanks.
In addition to the above, blown head gasket/cracked cylinder head may also be causes, but after the water pump was modified the EGR cooler or heater matrix appear to be the main culprits at the moment.
I had the 184 ps VRS Octavia and had all of the same issues, and despite numerous pressure tests and hydrocarbon tests, at different main
dealers, and an VAG specialist, couldn’t replicate or identify the fault. For the likely location of the leaks, the coolant either exits the system into the exhaust side of the engine, or is expelled via the expansion tank overflow.
Mine was a 77k mile in 5 years, and required 2 x water pumps (one was a catastrophic failure, coincidentally on my way home 150 miles after a pressure test, and probably as a result of running dry), expansion pressure cap (did nothing), heater matrix flush.
I also had 100k mile, 5 year 150 ps FR using the same EA288 block with no issues whatsoever, so I don’t think that mileage is necessarily a factor. This will probably be confirmed by a number of owners with the 184 ps engine that has been trouble free over a similar age/mileage.