From the bit of knowledge I have on pH sensors, the whole thing smells like either (1) they built the sensor wrong, or (2) it's not compatible with the pH meter head. Especially if it's reading the wrong value consistently/repeatedly, across two cal solutions.The old one had essentially turned to dust. It had been stored dry for 5+ years. Also, I've already thrown it away.
And according to the Q&A on Amazon, it works for the MW101.
Send it back.