This proposed feature would play havoc with a party member's ability to backstab one another (competitive gameplay).
In one of Larain's demonstration videos they showcased a party member happening upon (and killing) another party member who was standing around wounded after finishing a difficult battle. ( https://youtu.be/-QGsmYQaM88?t=188
) Toss auto-healing into the mix and opportunities like that go poof.
Another example shown was Swen coloring a poison potion to look like a healing potion and sneaking it into Thomas's inventory. From players being annoyed that the game automatically uses (or avoids) such potions during post-battle auto-healing, to the game auto-healing up the damage from such a trick far quicker than a sabotaged and confused player might react, there's just all kind of issues that pop up here.
While I personally have no use for competitive gameplay, it seems to be a pretty large part of D:OS 2's design and needs to be kept in mind when determining how suggestions can impact balance.