Woodland Mansions have a generation requirement of being above ~y=60. There is no way to change this via datapacks, such as a field within the structure json when `type` is `minecraft:woodland_mansion`. Additionally, `/locate structure minecraft:mansion` will cause the game to freeze for roughly a minute when all conditions except this one are possible to be met, rather than the normal result of an immediate unable to find message when there are impossible conditions for a structure to generate. Ocean Monuments will also exhibit this behavior with `/locate` when the only requirement that can't be met is finding a biome with the `required_ocean_monument_surrounding` tag

Easiest way to verify is superflat worlds of different elevations: attached are datapacks each adding a superflat preset where mansions should be able to spawn, the only difference being that one has ground at y=-59 and the other at y=69. `/locate` freezing behavior can be observed in the low one where no mansions will generate, and will succeed normally in the high one where mansions generate as expected