It's the part you put in is only a PCIe x2 unit not a PCIe x4 which is why the lane width dropped.
The newest version of the SSD is hard to come by and it will be more expensive. So far Apple doesn't offer it as a spare only as a swap out (you need to send the old one back) and it needs to be the same. Hopefully this control will end soon as the newer models don't use the same blade type of SSD. In any case it will be costly!
Adam you'll need to get into the system profiler service to check the new SSD's firmware comparing it with your older SSD.
Open About This Mac and click on the System Report button. Click on NVMExpress and SATA/SATA Express sections, enlarge the window so all of the info on the right is visible snap a screenshot of each and post it here so we can see what each has.
As a reference review this question and what Eduardo encountered: Upgraded my SSD, Was it not suppose to keep same read/write rates?