The issue here is that end_of_file for encrypted temporary IO_CACHE (used by filesort) is updated
using lseek.
Encryption adds storage overhead and hides it from the caller by recalculating offsets and lengths.
Two different IO_CACHE cannot possibly modify the same file
because the encryption key is randomly generated and stored in the IO_CACHE.
So when the tempfiles are encrypted DO NOT use lseek to change end_of_file.

Further observations about updating end_of_file using lseek
1) The end_of_file update is only used for binlog index files
2) The whole point is to update file length when the file was modified via a different file descriptor.
3) The temporary IO_CACHE files can never be modified via a different file descriptor.
4) For encrypted temporary IO_CACHE, end_of_file should not be updated with lseek