A while back I did an SSD upgrade on my 15" MacBook Pro 2013. The SSD I'm using is an ADATA SX6000 256gb. I purchased the Sintech adapter to use with this drive and I had no issues installing MacOS Mojave or any of the sleep issues that I've seen people have online.
However, after a month of use my MacBook suddenly won't boot and all I can see is a folder with a question mark flashing. I thought maybe the new SSD unfortunately failed but when I installed it into my main PC it was able to recognize it without any issues.
What I'm wondering is if this is an adpater problem as the original drive still works with this MacBook, it's only the new one that doesn't.
I'm looking for help or advice on how I can narrow down the problem?
You have pretty much answered your own question.
This points directly to the only other piece of hardware you haven't tested elsewhere, the adapter. So, unless you can test that adapter and drive in another similar Mac I would say you should replace the adapter.
Note that I have been doing computer support (Macs, PCs) for a long time and have learned that nothing is always as simple as that. Your issue seems cut and dried, and if I was a betting man I would bet on the adapter being the culprit. But computers are complex beasts and don't always yield to logic. They're too complicated, too many parts interacting in too many ways, for everything to be bug free.