Why does the Pokemon holding Power Weight lose HP when it didn't take damage in battle in Ultra Sun?

Question

In Pokemon Ultra Sun, I encountered times where Pokemon I was EV training in HP via SOS chaining wouldn't have full HP, and I never send out the Pokemon I'm EV training HP EVs for, so it doesn't take damage in the battle. However, after the battle, I check the Pokemon's summary and notice it won't be at full HP despite me not having it out in the battle. The Pokemon I don't send out holds Power Weight to get HP EVs faster. This happens when I do this with lower leveled Pokemon. This happened multiple times now, so here are the steps that lead to it happening:

1. Give the Pokemon (usually level 1 since I breed the Pokemon for competitive) I want to EV train in HP a Power Weight
2. Start an SOS chain with a Pokemon that gives HP EVs (i.e Wishiwashi, Alomomola, etc)
3. Have False Swipe Pokemon defeat 14 of the Pokemon that gives HP, giving Power Weight holding Pokemon 252 HP EVs. Do not send out the Pokemon that has Power Weight
4. Check Pokemon's summary afterwards and notice the HP thing.

So, is there a reason for the Power Weight (EV trained in HP) Pokemon to lose HP when it's not in battle? Is it some sort of EV and level interaction, or is there some other reason?

Comments

I actually think I know this one.

Say a Pokémon you're training levels up mid battle, and it gains the +2 or whatever to its Max HP. If that Pokémon was at, lets say, 75/100 HP, it would then change to 75/102HP.

I think the same thing may be happening as the EVs increase your idle Pokémon's HP stat. It will be at full health, then minus whatever health it gains from the EV boost.

I think...

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That shouldn't happen NobleOnion. When a pokemon levels and increases its max hp, that hp gets added to its current hp as well, so it would go from 75/100 to 77/102. So if it is at full hp then it should stay at full hp. Maybe there is a different interaction when the pokemon isn't sent out, I'm not sure what else could cause the pokemon to lose hp.