Because I've run into a situation where the barrels on an rca IC pair didn't fit the Jacks on my Hegel H390 (several other cables fit just fine), I'm inclined to give the dac builder the benefit of the doubt but I can't definitively prove whether it's the dac or the adapter that's to blame in this case.
I'll just have to see how the adapter that I'm expecting to arrive later in the week fits. Hopefully, it won't continue to be be an issue, going forward.