I use openSUSE Tumbleweed with KDE Plasma 5 (X version, not Wayland).
LibreOffice is from the official repository.

This problem has been there for more than one month (maybe two or more).

Procedure to produce,

1. Create a new spreadsheet in Calc,

2. Save as .xlsx format and check "Protect with password",

3. Close the document,

4. Open the saved document through "File" --> "Recent Documents" and input the
password,

5. Open KSysGuard to watch the increasing memory usage of soffice.bin

This issue also occurs within a newly created system account.