1. Has anyone seen or noticed this?
2. What card stock was it printed on? How many types do they use and for what?
3. They say they are still legal in tournament play, but being different card stock what is the chance that the cards could be cut to too easily during a deck check or such?
1) No not seen/noticed this at all. But then I have had a chance to look that closely at many Ixalan DFCs, so my sample size is rather small.
3) I would say that the chances of that are basically negligible/non-existent . As being DFCs, in a tournament you need to either use the Checklist card or use sleeves. So with the Checklist cards there shouldn't be any differences at all with the card stock. And in sleeves I would say that any noticeable differences, if there even are any, would become even harder to pick out.
So overall, I don't think there is anything really to worry about. Of course, I would expect Wizards to be doing a very thorough investigation into what caused this, so they can make sure to avoid it in the future.
But that is just my opinion on the matter, make of it what you will.
Alpha Investments or whatever he calls himself has a recent video on some new "fakes" someone sent him. Without additional information, it's up in the air on whether they're actual fakes or legitimate prints. I bring this up because the video talks at length about the hologram stamp.
Even though it would apply only to Ixalan DFC, it's one less tool we have to determine authenticity. How big of a problem would it be if any one DFC becomes valued?