18w20c new world. Maps sometimes show dirt and stone e.g. on a beach instead of sand, in cases leaves are skipped and the map contains a straight line (chunk boundary?)
(also sometimes rectangular chunks of forest are without trees, as can be seen, but that will be a different bug)
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MC-129623 is old world updated to 18w20, this was a new world. i think they're different issues

Resolving as duplicate of MC-129500

yes
Probably the same cause as MC-129623?