As title states. Once I exit combat it goes back to the position I set using edit mode.

I'm trying to find a way to recreate your issue. Can you show me your bag bar settings?

My bag settings is just like yours, but I have "Hide" checked. Also combined bags with "clamp to screen".

Is the problem with your bag bar or the backpack?

I guess both since it's combined. When I'm in combat the backpack is positioned at same distance from bottom of screen as your screenshot. I've moved it all the way to the bottom and right edge tho, and that's fine outside of combat.

So for the backpack, I can't move it while you're in combat. When you close and open the backpack, Blizzard moves the backpack back to its default position, which is usually either the bottom right side of your screen, or anchored to one of your action bars on the right of your screen. If you're out of combat, my addon will quickly move the backpack over to your custom position you set during Edit Mode, but if you're in combat, my addon can't do anything so the backpack will stay at the default position until you close and reopen it outside combat.

This is a limitation of the UI I unfortunately cannot do anything about!

No problem! At least I know why it happens.