I received feedback from PayPal this morning and this is what they tell me...
I was able to pull the logs and see that you are correctly passing the L_DESCn line items. Unfortunately, the problem you're experiencing is being caused by some technical issues with the PayPal system. Our engineers are currently working diligently on a solution to this problem. I am going to assign this ticket to our internal engineering ticket so that when the problem is resolved, you will be notified of its completion right away.
I apologize for the inconvenience and thank you for your patience.
Global Technical Support
At least we now know it's on their side and not the code. I'll report back once they figure it out.