The cause for a fluid mix is the power transfer shaft seal failure allowing the high pressure trans fluid to be forced into the gear oil case . Then causing an over fill in the differental case , then once the car is shut off the two cases balance back out . Results gear oil making it's way back into the automatic fluid .
It doesn't take long , a few warm ups to delute the gear oil & contaminate the trans fluid . I've seen this happen to old & current VW automatic trans .
Now for the real problem , the old auto's used two seperate cases for this setup . Before to fix the problem all you had to do was seperate the to units and fix the seal then reassemble the thing and your were off .
Now there is only one case with a single way to get to the seal , take the entire trans apart to get to it . So if it fails now it's a real problem to deal with .
And like I said I have first hand experience with this happening to new ( curent ) models without warning . In fact on the one I saw VW kept the car for a month then took another month to get a free replacement on the trans rather than the dealer tring to fix it . The dealer went back and forth over what they were going to do before they gave in and replaced the thing . They also tried to claim there was a problem in valve body , then the electronics , then the fluid was installed improperly , all claims proved to be wrong .
So don't rule out a fluid mix out of hand . Hope not , but could because this is one of the ways an auto acts when the fluid mix has taken place .
Good Luck,