My first xM1+ (installed August 13th 2018) stopped responding last week.
It had worked fine in the past, and I had it formatted for NDEF. It was readable with my iPhone, and I could read & write with my Sony android phone, and with my ProxMark3 Easy.
Last week I went to scan it with my android phone and nothing happened, like there was nothing there. So I tried with the iPhone, and same thing. As if there was no tag. Today I got the proxmark out and checked with that, same result. No tags found.
When I use the hf tune function, I can see the voltage drop as expected when the coil is over the tag, but it seems the chip itself has died. 
Considering I haven’t managed to get my defunct xEM out of my left hand yet, I doubt the xM1+ will be extracted any time soon.
FWIW my 2nd xM1+ is still working ok, as are both my xNT tags.
Ah well, we knew these were experimental or whatever.
sorry to hear that. If we find a suitable source for stable, reliable sector 0 writable Mifare Classic chips, we will make them again.