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.

[20:35:33] [Server thread/INFO]: 99% completed (291898 / 291987 chunks)...
[20:35:34] [pool-4-thread-1/INFO]: World optimizaton finished after 3635644 ms
[20:35:34] [pool-4-thread-1/INFO]: ThreadedAnvilChunkStorage (DIM-1): All chunks are saved
[20:35:34] [pool-4-thread-1/INFO]: ThreadedAnvilChunkStorage (world): All chunks are saved
[20:35:34] [pool-4-thread-1/INFO]: ThreadedAnvilChunkStorage (DIM1): All chunks are saved