Lost contains (about 1kb, 1024 bytes length) randomly in EOF

SYSTEM: Microsoft Windows 10 Professional [10.0.19045.6093]

When I open and edited some long files (128kb or longer), and save, the contains sometimes will lost (from EOF, and backwards about 1024 bytes length), or sometimes normally saved, without any lost.

I wonder how and why the contains near the EOF could be lost randomly?

No source file, test, or entry point is named. Start by reproducing the save operation on Windows 10 with edited files of at least 128 KB, checking whether roughly 1024 bytes near EOF disappear; done means the saved contents are preserved reliably.

Written by the indexing model from the issue text.