@Coto maybe the following could help:
I get the same issue with a R4 Gold Pro 2016 flashcart running YSMenu, on a DSi XL or new 3DS XL.
I either see a white top screen only or both white screens when loading the cart from the official 'home' app for each console.
The result occurs for the DSi XL and new 3DS XL with a couple of different loaders (YSMenu, TwilightMenu++).
I also tried using Moonshell2 (not sure if this matters) on the DSi XL, but nothing changed.
On attempting to load TGDS-multiboot (v1.6), I only get a top-white screen on the DSi XL and new 3DS XL.
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With a DS phat, the top-screen remains white, but the menu loads. On selecting a rom, the top screen remains white and the emulator seems to freeze. The result is the same if running the emulator via TwilightMenu++ or TGDS-multiboot (v1.6) from the flashcart.
With the DS phat, if I use TGDS-multiboot and select a rom as an argument, the emulator reports that the rom is being loaded, I get a sound pop, and then the emu seems to freeze.
I do not have a DSLite to test.
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Just to say, I used the arm9 builds in all cases.