Tl;dr: What would happen when the overflowed stat of any Pokémon would be lowered by any stat-lowering move, such as Screech?
About Overflowed stats,
A discovery has been made in Pokemon Sword and Shield which introduced a limitation. Pokemon in the game can naturally only have a maximum non HP stat of 654. If a Pokemon's stats overflow past this, they reset from 0 and add each additional raw stat point above 655. For example, if a Pokemon has 656 in a non HP stat, their non HP stat becomes 1. If they have 659 in a non HP stat, their stat becomes 3. If they have 780 in a non HP stat, the stat becomes 126, etc. In order for a Pokemon to have their stats overflowed, they must have a base stat higher than 248 on any non HP stat and a beneficial nature on this same stat.
So let's say that I have an Eternatus-Eternamax with 240 Defense EV's and a Bold Nature. This makes its Defense stat 655. The opposing any Pokémon uses, say, Screech. So would the Eternamax's Defense stat, which is 0, be lowered into minus numbers or something else? Or do they revert back to the original 655 and then get lowered?
If they go to minus stats, then won’t the Pokémon be worse than Sunflora in Defense (or Special Defense), or if the Attack (or Special Attack) gets lowered, then won’t every move deal <1% damage? Or do they deal an immense amount of damage? (This (attack overflow) is virtually impossible btw, but if you can get an answer for it, that'd be quite good.)
And what about Speed? Does it also goes to 0 and boom you outspeed everything or underspeed everything? (This (speed overflow) is also virtually impossible btw.)
Also, I dunno if this fact will help or not but with a 0 stat in Def or SpD, the Pokémon becomes invincible to the moves of the respective category (Def or SpD) in which their stat is 0.
This question has been bothering me for quite a while, any help would be appreciated!
I’ll add more info if I get some.