Ocean Monuments generating water in lava oceans worlds

I generated a customized world with lava oceans, ocean monuments are surrounded by water.

What I expected to happen was...:
Either the ocean monument was filled and surrounded with lava, either a border between lava and water

What actually happened was...:
The ocean monument was not filled and surrounded with lava and there was no border between lava and water

Steps to Reproduce:
1. Generate a customized world with lava oceans
2. Find a monument ocean

P.S: This couldn't be a bug, since ocean monuments filled with lava would be pointless, but I was at least expecting a border between lava/water.