Recently my 12-speed Shimano CN-M8100 chain running on a road bike broke with about 4,000 miles on it. One inside plate broke. So, the chain did not completely snap. Just started skipping weird in 11th and 12th gears.

I'm just curious if anyone has any thoughts on this. I'm not a powerful rider, maybe 800 W max sprint effort, and that is rarely. I know how to properly shifts gears, but of course occasional something happens and a shift may be a little clunkier than desired.

Here's more info in an attempt to answer questions before they are asked.

The chain came on new bike and therefore not likely to be counterfeit. When new, I stripped with Silca stripper per instructions and then used Squirt drip wax. No degreasers that would cause the metal to get brittle.

Of course, I understand that 12-speed chains are thinner than 11-speed, which are thinner than 10-speed. But I don't think it is coincidence that it failed at the master link (see pic). Though there's nothing unusual on the inside of the master link, such as burr or similar that would have worn on the broken plate. Plus the crack doesn't follow a curve like I would expect if the master link were wearing on it. No measurable chain "stretch" using a proper measurement tool.

While the chain is dirty, it's just blackened wax. It's not gritty at all. And what looks like a copper color on the inside of the master link is a reflection. That surface is polished very smooth and is still silver in color.

This is my first 12-speed road bike, so I don't have experience to draw on. Is 4,000 miles reasonable for a chain to break? Do these tend to break at master links? Bad luck, or do I need to replace the chain on say a 3,000 mile schedule. Or periodically do an extreme examination in an attempt to see a crack before it becomes a full break.