I was a job today ( I thought I was done with this) and saw two tubing brands that I suspected were not barrier tubings. I tried to see the name of the red one and I think Infloor was the brand? Just dunno.
The other was a a dull white with no markings. I know this is not a lot to go on. The heat source is bradford white water heaters and have failed twice in the last ten years. I know, great system. It's the tubing thing that's first though.
should not cause a glass lined WH to fail. Plumbing pex is used all the time on water heaters and they see a daily influx of O2, every time you run a faucet.
Hydronic expansion tanks are usually the first to fail from O2 ingress on heating systems.
Are the tanks leaking, or corroded from continuous periods of condensing mode operation. Could be they are corroding from the outside in. How do the burners look when they fail? Are they covered with rust particles?