While looking through the files of the game to make a data pack, I noticed that the Cartographer had its trades be biome specific for its banners. They go as the following:
Plains - Brown, Pink, White, Yellow
Desert - Cyan, Gray, Green, Lime, Orange
Savanna - Green, Magenta, Orange, Red
Taiga - Blue, Lime, Pink, Purple
Snowy - Blue, Cyan, Light Blue, Red, White
Jungle - Brown, Green, Yellow
Swamp - Black, Light Blue, Purple
However, there is no code file that allows them to sell light gray banners, effectively making them impossible to get from that specific villager.
This is working as intended, reading the 25w07a snapshot notes, the new biome-dependant cartographer trades are shown in the image attached, where the light gray banner is explicitly omitted from the trade pool.