If you are using a "pre-patched" or "trimmed" version of the ROM, you must ensure the update matches the specific version of your game file, or it will not work.
Digital Preservation and Distribution: A Technical Analysis of the Pokémon Ultra Moon Update 1.2 CIA Patch in the Nintendo 3DS Ecosystem pokemon ultra moon update 12 3ds world cia patched
Verdict: As of 2025, Nintendo has shifted all resources to the Switch 2. The 3DS ban waves ended years ago. If you need v1.2 and cannot download it legitimately, the patched World CIA is the only viable solution. If you are using a "pre-patched" or "trimmed"