SUMMARY
When multiple instances of Okular are open, the recently open files history of the last closed instance will override all other ones.

STEPS TO REPRODUCE
1. Open two different files (say, 1.pdf and 2.pdf).
2. Close the instance showing 1.pdf, then close the instance showing 2.pdf.
3. Open a new instance of Okular.

OBSERVED RESULT
Only 2.pdf will be shown on top of the "Files -> Open recent" menu.

EXPECTED RESULT
Both 1.pdf and 2.pdf to be shown (preferably ordered by "last opened topmost").

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux with kernel linux 5.11.10.arch1-1
KDE Plasma Version: 5.21.3
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2

Tested on both Okular 20.12.3 and Okular 21.07.70

ADDITIONAL INFORMATION
First timer, I would like to start contributing to KDE; if this gets confirmed, I would like to give it a shot and try to fix it myself.