No, I don’t think you’re missing anything. What is happening is that the right channel is silent I think.
This behaviour replicates here. It also chimes with my very first comment about mono plugs in stereo channels - although I wasn’t specifically referring to Amp Rack in that case. I think that I have usually used Amp Rack only in mono configuration and not had an issue, although it’s nagging me that I have seen this behaviour before.
Why it plays, but won’t render is definitely one for Steinberg’s wizards.