When loading a world in the snapshot version 19w14b, everything is fine and chunks are not overwritten. After saving and exiting, then opening the same world in 1.14 Pre-release 1, some of the chunks are over written.
Environment
Running Java 8 Update 201 and Java SE Dev Kit 11.0.2
Windows 10 64bit
Intel i7-7500U CPU
Duplicate of MC-147800. Please use the search function before reporting.