No idea, but this thread suggests there can be problems with some non-oem ones http://www.ozmpsclub.com/forum/diagn...-downpipe.html
Doesn't look like a common cel (yeah, 2nd o2 cels are super common). Since its pre-cat, it sounds like an actual problem with the sensor than with the downpipe. All the trouble shooting I can see is about checking wiring of the sensor (e.g. https://euroesi.mazda.co.jp/esicont/...2b7813300.html) and since they were replaced, that is where I would start.
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