There are other SCR diesels that have had this problem, too. Seems to be pretty common. My suspicion is the same basic construction is used on all of them, regardless of vehicle manufacture, and therefor they have a similar problem.
I would have thought that by now, SCR and DEF use would have ironed out most of these kinks, but it so far appears it has not.
Good news is, so long as it remains a tiny amount, it isn't likely to do any harm (and it rinses away easily with water... just do not do so while the engine is hot).
VAG does have a TSB about this, though, so maybe they actually HAVE figured out a solution to it... in this case, they say a revised supply line is the fix.
So at your earliest convenience, I'd take it in and let them look. You may want to call ahead and ask if they have the line in stock, and if so, arrange to drop your car off the night before, so it can be cold and they can fix it for you the next day.
2015 Jetta's line: 5C0-131-984-B