In all versions past 1.14, old customised worlds from 1.12 can be opened but will generate the default terrain. In 20w21a and 1.16 when customised worlds were added via "import settings", they don't show up in the world selection screen.
Comments 3
This issue is not within any specific world but appears in the UI. THe world types go from Default -> Superflat -> Large Biomes -> Amplified -> Single Biome and then back to default, skipping over Old Customised.
Can you please attach a copy of such a world?