I'm 100% certain the plate was broken early in that ~70-mile ride if not before the ride. The reason is, the symptom was a sort of chain skipping clunking happening in 11th and 12th gear. I thought perhaps the derailleur hanger got bent somehow. I even stopped to make sure the derailleur simply hadn't come loose. It's a Di2 system, so a cabling problem couldn't explain the issue. It wasn't until I got home and on the repair stand I was able to immediately see the chain lift and then fall coming off the 11 th and 12th gears. The reason for this is the larger part of the broken plate had rotated such that it was interfering with the smaller cogs and causing the chain to lift up a small amount. Then of course, once this broken link cleared the cog, the chain fell back down into proper position on the cog. This made for the skipping feeling and clunk sound.

I think it is likely the plate broke a week or so earlier. I developed a slight sound in the rear as if the rear derailleur needed adjustment. Being Di2 this is not expected. But a single indexing step eliminated the noise. So, I believe the chain was broken at this point causing it to track differently on the cassette. It wasn't until the broken plate rotated that the more obvious problem developed.

Fortunately I wasn't doing any max efforts out of the saddle, as perhaps the chain would have snapped and I would have fell.

However, I don't think this says anything about how marginal the chain may be. That 70-mile ride was specifically a zone 2 ride, so it was very much on the low-stress side of what a chain will see. I think I can say with certainty that this single plate, carrying the full chain tension, saw less stress in this time period, than it has at other times when both plates were sharing the stress. Also, my max power is less than half what a powerful rider could do. So, even a single plate wouldn't likely just fail at the power I'm able to produce. Of course, once broken, the remaining plate sees a range of different forces not just that it carries all of the chain forces by itself.

If nothing else, in the future, I'll be more wary of a chain issue if anything seems off. While I don't know that I would have spotted this break easily. It's still worth trying to spot this type of thing in hopes of catching it sooner rather than latter. If nothing else, it could be worth examining the inner plates at the master link position periodically. And I may just be extra cautious and replace the chain every 3,000 miles.