[Bug]: Desert Coverage below 50% on a height 1 map does not spawn any desert tiles.

Version of OpenTTD

15.3 Windows 10

Expected result

Attempted to create a 512x256 world with 40% desert coverage. (Also tested with other sizes and coverages less than 50%) No GRFs.

Actual result

Entire map was forest, with no desert tiles.

Steps to reproduce

Simply generate new world with 40% or less desert coverage.