This may go back to maybe all versions of LibreOffice and even OpenOffice.org.

When opening a password-protected document, the word completion tool, which is on by default, automatically collects a large amount of alphanumeric information into the word completion list, which persists and remains accessible even after the password-protected file is closed.

At the same time, the "Remove collected words when closing a document" option is off.

For better privacy and security, I recommend, that LibreOffice:

* will not automatically collect words from password-protected files,
* not from digitally-signed files (increased information sensitivity may apply); and
* will not not save these words anywhere in storage;
* should automatically remove word suggestions collected from password-protected files, if a user has chosen to collect words during the document open session for word completion.
* Possibility should be a deny-all option, and a permit-narrow-scope option with checkboxes.

* Maybe also an option to encrypt words collected from password-collected files in RAM memory.

These options could then also be extended to files opened from encrypted disk volumes, and specific cloud sources that may require enhanced security.

The current workaround is only to turn word collection off, while I want to keep it for some open files, and not others.