Bug 461873

SUMMARY
***
when opening files from dolphin or command line, opening another file lunches another window instead of opening in another tab on the same window despite the open new files in tabs option being enabled.

selecting multiple files and opening them or passing multiple files on the command line does open the files in tabs under the same window.
***


STEPS TO REPRODUCE
1. open pdf file from dolphin or terminal.
2. open another pdf file from dolphin or terminal.

OBSERVED RESULT
each opened file launches a new okular window.

EXPECTED RESULT
all files will be on the same window under different tabs.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: EndeavourOS
(available in About System)
KDE Plasma Version: 5.26.3
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.7

ADDITIONAL INFORMATION
I checked all okular related .desktop files in /usr/share/applications and none has the `--unique` argument passed so that's not the issue (that was the case on https://bugs.kde.org/show_bug.cgi?id=428923).
this issue (https://bugs.kde.org/show_bug.cgi?id=436774) seems to be the same but on windows.