Most likely you are running into the issue from forcing Single-Ended (SE) mode on a Low Voltage Differential (LVD) bus.
The SCSI architecture for the Fuel should be running at least Ultra160 LVD (if not more). These are Low Voltage Differential signalling buses. Now for many SCSI adapters (that also have interactive firmware, like Adaptec). You could FORCE an LVD bus back into the Parallel SE mode. This is what 50-pin (Narrow) and 68-pin (Wide/Ultra-Wide) are. You CANNOT mix the two, a SCSI chain runs at the slowest bus standard of all devices present on it. So if you put an LVD device and an SE device on the same chain...they both need to run as SE (good luck with that in practice).
However (regardless of SGI), I've seen embedded SCSI controllers for servers that were LVD-only and absolutely threw a fit when you tried ANY SE devices. They hated it, refused to do it, and generally were considered LVD only. I assume the last SGIs were configured this way.
The adapter you got IS SE. Because it has 50 pin, this isn't an issue with active termination of the 68->50 pins...you're attempting to downgrade the actual SCSI signaling standard to an obsolete one (in terms of the Fuel's perspective).
You need to be use something like this: https://cs-electronics.com/product/adp-9018-sca80-68-lvdse-for-ultra23-and-ultra160320-1-form-factor/
Please note the need for this kind of thing is RARE, the need to change a SCA drive to Ultra160/320 68 pin connector was just was never an expected thing. So I understand your issues, but while SCA drives are cheap, that's not what the Fuel takes (which is odd, considering the other SGIs DO take them). So be extremely careful not to mix SCSI standards. You won't blow up anything, but you're forcing the controller to a whole other signalling protocol...and even thought it should work, I'm not surprised it's throwing a fit. Many Ultra SCSI controllers were not really tested for SE and many either have intermittent problems or just outright refuse to work on an old SE device.
My advise, either buy a normal Ultra160 HDD (or get the right adapter, if you can) and then put in a AHA-2940UW card in the FUEL to run external and internal SE devices off that card. (since that's the correct standard for that adaptec card).
I've seen entire SCSI LVD array enclosures that worked ONLY on LVD and any SE devices caused firmware crashes. It's a thing.