Hey there. Your code looks fine so looks like a bug. Could you potentially upload that particularly Region file somewhere so I can try to reproduce it?
I've released fastanvil 0.14.1 that should fix this.
This seems to be due to one of the chunks in the region not being padded to a multiple of 4096 bytes as the wiki claims it has to be. The chunk metadata includes to proper unpadded length, so I now use that instead.
The following code:
gives this error and no other output
I've tried with 1.15 and 1.16 region files and had no luck.