I know why Tesla made the change, and that's because alcantara has become too expensive for Tesla and they're cheaping out. When you think about it it's the only logical thing:
They removed alcantara from the Model S and X dashboards some time ago. Why would they do that? It was generally well-liked. It's known that there is a near monopoly on alcantara, there are only a couple suppliers and arguably only one in North America depending on the article. It's also known that demand for alcantara has gone through the roof (this is talked about in numerous articles) due to its increasing use in cars, and consumer electronics like tablet cases (think the alcantara on the side of the ipad magnetic screen cover).
While Tesla wasn't cut off, the supplier surely raised the price for them and probably other purchasers, so in turn Tesla cut down its use on the dash's. Now apparently the price increased a lot, and Tesla isn't down for any of that, so they bailed on it altogether. Tesla's cheaping out is why they needed to spin/lie about it, if it was truly a vendor cutting them off I'm confident they would have said it.
Important to note: Elon Musk has said repeatedly on his shareholder conference calls that Tesla has been must stricter with suppliers for Model 3. He SPECIFICALLY mentioned that any suppliers who fail to deliver or perform on time would face large penalties per the contracts Tesla was drawing up with each supplier. For this reason, it's incredibly unlikely that it's a supplier issue, and very likely it's a decision on Tesla's side.