Hi @Nguyen24174130fz5j,
Thanks for the message,
This is an interesting question. The mask thumbnails depend on multiple things. First of all, if you are working with UDIM, Substance 3D Painter will automatically simplify the thumbnails (for optimisation).
On the other hand, if you're not working with UDIM you can indeed switch options under Edit>Settings>General>Layer stack options>Use simplified thumbnails
I can see on your second screenshot that you're working with UDIM, so it's perfectly normal.
Have a nice day.