We do that in a boiling water reactor. But in that case the water is the coolant and the moderator, so as you boil, you get less moderation, which protects the upper portion of the core. We also “shape” the flux profile by adjusting enrichment and gadolinium content (burnable poisons) in the fuel.

Normally in a BWR, power leaks in the bottom 1/4 of the core, and as you deplete the fuel in the bottom later in the cycle, the water is able to “climb” further up the core before boiling, which improves the moderator in the upper portion of the core. By the end of core life, the power peak is in the top 1/3rd of the core.

So it can work when designed right.

But yeah in nearly all other cases, you want to keep your coolant and moderator in a single phase (for the most part)