Many Listings vanished?
I remember reading somewhere that they were gonna clear out the listings that list prices lower than the vendor price. It could be that.
Currently the trading post only shows the 20 lowest sell orders for any item, so if the items you’re looking at only have a few items at each price, then supply displayed will be much lower than actual supply.
Also, when you first search for an item that has 1000+ quantity, it will properly display that, however, once you click on an item to show it’s detailed listing info, if you return to the search results, it will retain it’s lowered supply number.