And another one for the list.
I couldn't believe the horrendous racket coming from the chain area because of that sodding little ring
I think I know why they break.
Check out the wear grooves in the VVT adjuster. I am fairly certain the plastic rings catch on those steps and snap. I think it could also be exacerbated if you don't slide the chain housing back on absolutely dead square, which I probably didn't do the first time around
Now it becomes clear as to why VW sell a new housing, VVT adjuster, gasket and the bolt as a kit. I always thought replacing the VVT phaser was optional when doing the chain, but it appears to be mandatory otherwise the bloody rings can snap. 160K on that phaser for reference. Shame to have to replace it as it looks fine, but it's fooked unfortunately.