Did someone say analogy?
The seal on the glow plug is the same concept as the prechamber seal. The implementation is nearly identical to that of a flare fitting in plumbing. You have a tapered seat that a machined fitting is pressed against for a tight metal-metal seal. They work very well, but are critical that the sealing surfaces are clean and the connection is tight. See this picture for a visual on a flare:
The actual "flare" is in the cylinder head and the "fitting" is your glow plug threaded into it. Any rust, carbon, or other garbage left on that sealing face will prevent a good seal. Over time the seat will start to leak just from differential expansion and having all that hot exhaust gas pressing against it. Eventually some "gunk" will get out and get the glow plug stuck. It hapens on the earlier engines too, but with their short glow plugs, you don't have to worry too much about torquing them in half. With the long skinny plugs used in the 606, it's more of a problem. There have been threads on here on how to grind off the threads from a glow plug and cut a notch in the sealing face to follow up behind a glow plug reamer to re-dress the seat.
I'm personally not a fan of VW products. I don't like the way they drive and I don't like the way they're engineered (they seem to have way too many expensive problems). For the cost of the car when new, you'd expect better. The newest VW I've driven was a 2014 Passat with the 2.5 5 banger. HATEFUL car. Uncomfortable driving position, a truly gutless engine, transmission that always wanted to be in 5th gear even when stopped, steering that felt like grabbing a marshmallow, and a suspension that managed to be both soft and sloppy AND choppy at the same time - have NEVER experienced that before! I've been spoiled by driving decent cars, so my standards are probably higher than they should be, but I haven't and won't own a VW.