Why doesn't the Earth's outer core dissolve into the mantle?
From the comments:

And indeed, after browsing trough a few sections they seem to assume that &quot;Liquid metal separates rapidly from liquid silicate&quot;, like oil and water, and they don't mix later. Very interesting. I wonder what's @Gimelist opinion about this.

You are very correct.
The answer is simple: the core and mantle are immiscible. The core doesn't dissolve into the mantle because it's not soluble in the mantle.
This is precisely why we have the core and mantle to begin with.
My answer here is very relevant: https://worldbuilding.stackexchange.com/a/123951/39999

Let's take a hypothetical very simple core and mantle composition:
The core is made of pure iron ($\ce{Fe}$), and the mantle is fayalite ($\ce{Fe2SiO4}$).
What happens when you chemically react the two by $\ce{Fe + Fe2SiO4}$? Absolutely nothing. There is no intermediate $\ce{Fe3SiO4}$. Does not exist. So they just stay as separate iron and fayalite. And because iron is much denser than fayalite, it stays down at the core, whereas the fayalite is the mantle above it.

Edit:
Here's a recent paper describing the overall redox conditions of Earth compared to its neighbours:
https://doi.org/10.1038/s41467-020-15757-0
May be of relevance.