If an Ocean Monument spawns in a deep frozen ocean biome, the passageways that are usually filled with water can be clogged with ice. Given the mining fatigue effect and the slower water mining speed, this pretty much dooms players trying to explore one of these monuments.

The seed -2047330036221477911 has this bug at the coordinates 1609, 44, 107891.

The ice seems to only replace the water and not any of the prismarine blocks. I'm not sure how monument generation works, but if the generator creates monuments after the icebergs, then the solution would be to force the monument to generate water within its passageways and replace any blocks in its path. If the monument generates before the icebergs, I can only assume that icebergs only replace water with types of ice, regardless of whether its inside a monument or not.