If you are using a modified 3DS with Custom Firmware (CFW) , there are specific considerations for installing the update via CIA:
Resolved a rare case where the game would crash if an evolution item was the last item in the bag and was used. Pokemon Sun: Update 1.2 3DS (WORLD) CIA
Update 1.2 for Pokémon Sun on the 3DS is a significant patch released to address critical bugs and is required for all online features like trading and battling. If you are using a modified 3DS with
Fixed a bug that sometimes prevented Scatterbug from properly learning Egg Moves. Pokemon Sun: Update 1.2 3DS (WORLD) CIA