Select chunks reset to world seed upon loading a world from a previous snapshot

Upon loading a server world from 18w16a in 18w19b, I logged in to find that various chunks had been deleted and replaced with default (flatland) terrain generation. In the server log are java error messages and server error messages as following:

[09:59:39] Couldn't load chunk
java.lang.NullPointerException: null
at bqt.<init>(SourceFile:70) ~[minecraft_server.jar:?]
at bpl.c(SourceFile:627) ~[minecraft_server.jar:?]
at bpx.a(SourceFile:455) ~[minecraft_server.jar:?]
at bpx.a(SourceFile:150) ~[minecraft_server.jar:?]
at bpx.a(SourceFile:98) ~[minecraft_server.jar:?]
at si.g(SourceFile:94) [minecraft_server.jar:?]
at si.b(SourceFile:118) [minecraft_server.jar:?]
at sq.<init>(SourceFile:39) [minecraft_server.jar:?]
at sr.c(SourceFile:192) [minecraft_server.jar:?]
at sr.c(SourceFile:284) [minecraft_server.jar:?]
at uv.d(SourceFile:350) [minecraft_server.jar:?]
at ti.a(SourceFile:561) [minecraft_server.jar:?]
at mk.a(SourceFile:126) [minecraft_server.jar:?]
at mk$a.a(SourceFile:57) [minecraft_server.jar:?]
at ir$1.run(SourceFile:13) [minecraft_server.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_162]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_162]
at k.a(SourceFile:149) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:666) [minecraft_server.jar:?]
at ru.w(SourceFile:364) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:622) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:527) [minecraft_server.jar:?]

Screenshots of the affected chunks are attached.