Woodland mansions do not generate if ground is too low

EDIT: Can this ticket have its status changed to postponed due to the apparently temporary removal of Customized worlds?

The bug

Certain presets appear to fail to actually generate woodland mansions in the world, even though the game recognises there should be one there. This contrasts with MC-117285 where the game does not seem to recognise any positions for mansions; attempting to find one via /locate would produce give an error indicating a failure to detect any mansions in that case, whereas here coordinates are provided for a mansion, the coordinates leading to a plausible location inside a roofed forest biome, but with no mansion in sight.

How to reproduce

1. Create a customized world with the seed 5589544208840976890 and the following preset:

   {"coordinateScale":1.0,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":317.838,"depthNoiseScaleX":1.0,"depthNoiseScaleZ":1.0,"depthNoiseScaleExponent":0.01,"mainNoiseScaleX":1.0,"mainNoiseScaleY":1.0,"mainNoiseScaleZ":1.0,"baseSize":2.859155,"stretchY":30.285635,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":20,"useCaves":true,"useDungeons":true,"dungeonChance":7,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useMonuments":true,"useMansions":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":100,"useLavaLakes":true,"lavaLakeChance":100,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":4,"riverSize":5,"dirtSize":33,"dirtCount":10,"dirtMinHeight":35,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":8,"gravelMinHeight":35,"gravelMaxHeight":256,"graniteSize":50,"graniteCount":40,"graniteMinHeight":0,"graniteMaxHeight":10,"dioriteSize":50,"dioriteCount":40,"dioriteMinHeight":3,"dioriteMaxHeight":12,"andesiteSize":50,"andesiteCount":40,"andesiteMinHeight":5,"andesiteMaxHeight":14,"coalSize":17,"coalCount":20,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":9,"ironCount":20,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":9,"goldCount":2,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":8,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":8,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":1,"lapisCenterHeight":16,"lapisSpread":16}

2. /tp -6056 64 8168

If the mansion instead spawns at a location much higher than normal (including areas which would cause it to be above y=256), the mansion generates anyway.