REMEDY HERE:  I had the same issue and was able to fix it.  In my case, the item was mapped to MISC and I needed it to go to NEC.  Rather than clicking on the option to map to NEC, I first had to click on the option to map to MISC, which does seem counter-intuitive, but hang with me.  The drop-down box allowed me to change the setting to "OMIT FROM 1099", so I did.  I saved and started over with mapping to NEC and the option to remap that item to NEC is now available.  I hope this helps you, too!