...not because they are new, but they happened to be the 'entry' level chipset of the pro psr series. also no touch screen, etc.
The A5000 is also the sx600 chipset (but with a lot added on); but it *does* use the hardware from the sx700/sx900 series (touch screen, more compatiblity with USB sticks). So even though it share the basic chipset, the motherboard is different, and it *can* read my 128gb usb stick.
I just forgot about the A5000 when I typed the above because we sold all 3 of ours almost immediately, leaving me with no floor model unit
And they do post compatiblity lists, which they update
https://br.yamaha.com/files/download/other_assets/2/1366042/en-PSR-SX600.pdfIf it makes you feel any better, it's more to do with the huge variety of usb sticks; Xbox Series X has the same issue with TV's; they claim 4k, but if your TV hasn't been added to the compatibility list yet, it'll only work in 1080 and below. (If you contact them with your TV's model though, they *will* add it)
In Yamaha's case with USB sticks, there are probably too many for them to add individual user's sticks by request. Just simpler to test ones that work (as they are released) and add to that compatibility list.