Looks like this has been reported recently on r/bitcoin, but I wanted to highlight that the issue is still present. I was trying to order an item on Overstock.com and Mycelium successfully retrieved the payment details from Coinbase's payment request URL, but could not reconnect to submit a signed transaction. FYI: /u/coinbase_vlad
I had to manually input the address and amount data into Mycelium and pay as if I was using the fallback URI. For those stuck in this situation, I recommend using QRStuff.com to quickly generate dynamic codes using any textual content: Click the Coinbase QR to get the "bitcoin:xxx" URI string (or right click as necessary) and then paste the string into the Plain Text entry box on QRStuff.com. It sure beats manually typing in the payment data to your other device!