0. Set auto-save frequency to 1 minute (for faster reproduction);
1. Open user profile's backup directory (e.g., on Windows it's C:\Users\<username>\AppData\Roaming\LibreOffice\4\user\backup) in a file manager to see its changing content;
2. Create a new spreadsheet; save it as XLS (97-2003) with a password (e.g., as "pwd_backup_proliferation.xls");
3. Make an edit to mark it modified;
4. Wait 1 minute and let auto-save to trigger.

In the backup directory, empty (0-byte) files start to appear ~every 5-10 s, named like this: pwd_backup_proliferation.xls_0.ods; pwd_backup_proliferation.xls_1.ods, ... etc. Closing the file does not remove the generated files.

The impact is not only proliferation of empty files in the directory, but also backup save not working for password-protected XLS files.

Reproducible with Version: 4.3.0.4
Build ID: 62ad5818884a2fc2e5780dd45466868d41009ec0
(there files were named slightly differently: pwd_backup_proliferation.xls_0ods)

and with Version: 6.2.3.1 (x64)
Build ID: 9ba025bafb03b962c34687cf87806cc03a3a7436
CPU threads: 12; OS: Windows 10.0; UI render: GL; VCL: win; 
Locale: ru-RU (ru_RU); UI-Language: en-US
Calc: CL

In Version: 4.2.0.4
Build ID: 05dceb5d363845f2cf968344d7adab8dcfb2ba71
it works OK: it creates a single file ~10 KB. Closing the file removes the backup.