To me it looks like it's just the load on the chain on that gear combo...pulling the chain away from the idler pulley. I think the reason it seems to track properly when pedaling backwards is because the load or pressure against the chain is from the bottom of the chain ring, not the top as it is when pedaling normally. I think the chain length is borderline a link to short which is the cause of the mis tracking. It's not a problem because it is unlikely you are in this gear often or for very long...such a degree of cross chaining isn't good for the drivetrain but most only do it for brief periods. Sometimes you just have to do it...
Try adding one link and see what happens.
You're not really hurting anything an in such cases most people in the same circumstance...chain length, gearing...while riding don't notice it because you are riding and not many put the bike in this gear while it's on a stand except to check chain length.