It should be sand instead. See attached screenshot.
How to reproduce
Create a desert surface buffet world with seed 1.
Go to coordinates:
/tp 60 59 68
→ ❌ Dirt is generated under the farm
Linked issues
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Comments

Can confirm for 1.13-pre10.

Can confirm for major release 1.13.

Can confirm for 18w32a.

Can confirm for 1.13.1-pre2.

Can confirm for 1.13.1.
This issue relates to MC-123312.