Affects Version/s: 20w17a
Fix Version/s: 20w18a
The height map data for chunks is not converted when a world from pre 20w17a is upgraded to 20w17a.
The height map data is encoded the same way block states are. There is a converter for the block state data, but not for the height map data.
Here's a world created before 20w17a (invalid height map data):
And here's a world generated in 20w17a: