Hm... so I'm missing something. Here what I did:
1. Exporting switch save using checkpoint and transferring it to PC.
2. Load the save at PKHeX and SAV > Items > +1 Pokeball
3. File > Export SAV > Export Main (same name as the original save)
4. Transferring it to switch and using checkpoint to importing it back.
What did I do wrong? Do I need some extra steps I'm not doing?
Using The Latest PKHeX. If I edit anything on my save and export it, Pokemon let's Go Eevee says the save file is corrupted.
I'm attaching the original save and the corrupted one (just did +1 Pokeball).