Am I experiencing an SSD failure? Or something else/worse?

[Audit13]

Yes, all of the other macs are updated and work fine with other apple SSD blades.

I tested it on a late 2013 15" rMBP (Originally installed in this one), late 2013 13" rMBP, two different 2015 15" rMBPs (2.8 no dGPU and 2.5 dGPU) and my 2013 nMP. All of them work fine with Sierra installed on the drive, but fail if trying to install High Sierra or Mojave on it.

Anyway, I'm not trying to troubleshoot the drive (already tried before) as it is only backup storage nown the usb3 owc case. I just wanted to add some info for the OP that might be related to the OP's ssd problems.

That's interesting about your drive.

I have a late 2013 13" Pro and it's running apfs without any issues.

 

[smbu2000]

That's interesting about your drive.

I have a late 2013 13" Pro and it's running apfs without any issues.

Yeah, I know. I’ve run Mojave on those late 2013 rMBPs just fine with other ssd drives. Only that one has a problem with APFS. Very strange. My nMP came with it’s own 1TB ssd drive and my current 2016 15” MBP has soldered in storage, so the “faulty” 1TB drive is a backup now.
I would have tried it in the 2015 MBA I used to have but the physical size of the drive is too big.

I thought maybe the OP might have a similar issue.