1.13.1 server - java 10
1.13.1 client - embedded java
Tested the conversion in both environments, same results.
I performed a forceupgrde / optimize world on my old 1.12.x world.
It caused random chunks to disappear. No errors were given so I thought everything went smoothly but when I inspected more I noticed chunks were missing.
See provided screenshot from minecraft overviewer, the chunks ARE explored in 1.12.x. and below, not sure in which exact version these chunks were explored tho.
Since the log didn't output any error, this is the only information I have.