After altering a Pokémon’s stats in the Json (Tested with Magikarp and Walrein) If you use the pokemon editor item the stats will revert to the standard if IVs or EVs are changed, the same also occurs to abilities if you alter the pokemons ability.

Hi, I have tested your issue and can confirm it to be somewhat true.
The changed Pokémon's stats do appear to be reverted reverted back to the standard ones in the editor. However, they do apply in the actual Pokémon (after a server restart). For example, secondary or hidden abilities will show if an ability capsule/patch are used.

However, it is correct that the editor doesn't display custom changes properly.
Will be moved to internal soon.

Thanks for your report.

Issue moved to internal; thank you both for your report!