ohh 5.6.6 is that out in the wild? from what I have read that was the most stable version that was ever released for the 3DS.mmmm on unityforn3ds 1.3.4 (unity editor 5.6.5) I haven't heard of a bug related to sprites or culling, but yes to the UI button object made from the editor...
aniway I'm not using that versión, i'm using unity 5.6.6 (unityforn3ds 1.3.10) and I have never had problems with sprites or buttons made with the unity editor, Only this is that when activating culling occlusion, the 3D objects (some cubes made with unity and a low poly house) and Sprites that move in the scene do not render in the camera when testing in the console or in citra, as if they did not exist.
I know they exist because the collisions of those objects work unlike rendering.
Anyway, I'll try what you suggest and see how it goes, thanks in advance <3