I went to check out some missions that had failed which show in the new assignment history section. Scrolling down, I find one of my missions had been successful, I had apparently sent my purple diplo-doff on it- and... wtf
This has nothing to do with the new UI. I already sent a ticket about it yesterday before the patch was live. It is probably a issue with completed assignments. The UI "remembers" not only the first but all Doffs of an assignment. If you have an assignment with 3 Doffs all 3 will be remembered as long as the next completed assignment you click needs at least 3 Doffs too. If the completed one needs more the additional Doffs will be the one you actually sent. Click on a completed assignment with less than 3 and the UI forgets the Doffs again except the amount needed for the assignment. Click on an active assignment and you see the right Doffs and the UI remembers a new set of Doffs.
So active assignments overwrite assigned Doffs-cache, completed assignments take over the existing Doff-cache.