The biome PalettedContainer data array length was calculated as
ceil(64 * bitsPerEntry / 64) which is incorrect for non-power-of-2
bit widths. The correct calculation uses SimpleBitStorage's formula:
valuesPerLong = 64 / bitsPerEntry, then ceil(64 / valuesPerLong).
For example, with bitsPerEntry=3: old formula gave 3 longs but the
actual data contains 4 longs (valuesPerLong=21, ceil(64/21)=4).
This bug was masked in 1.21.5 by excess padding bytes in chunk buffers
(due to PalettedContainer.Data.getSerializedSize over-counting). MC
1.21.6 fixed the size calculation server-side, removing the padding
and exposing this pre-existing MCC bug.
Made-with: Cursor