Look its simple, Bitmain/Zynq is cheap right? They are not going to put a chip in there capable of handling SDXC or more, when there are surplus of obsolete SDHC or older available. This is why 32g is the limit. BUT, there are 32g SDXC cards out there, which will, of course, not work. That is why they wrote that 16g max thing. More than 32g will absolutely not work, the border from SDHC and SDXC is right at 32g. Besides from their point of view, customers are almost never ever using the micro SD slot...

And of course this is disregarding the fake ones, which just complicate matters. Smaller sizes tend to be less faked. Braiins OS only needs 128m so its a huge waste of capacity anyway, get the smallest size you can