SUMMARY
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When you download something, for example in Firefox and try to open folder that contains that file either from Firefox or by using widget in the notification panel it opens up with one of 2 cases:
1) IF Dolphin is already opened AND doesn't have this folder already opened prior to you opening it with methods described above THEN new Dolphin instance is opened. Which is bad for reasons below.
2) IF Dolphin is already opened AND does have this folder already opened prior to you opening it with methods described above THEN Dolphin is switching to this tab. Which is good.

Why 1st case is bad? Not only it creates new instance as a minor inconvenience but if you don't have Dolphin already opened with your favourite tabs that are remembered the new Dolphin instance completely erases those. So you lose your tabs.

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STEPS TO REPRODUCE
0. Open Dolphin, open few tabs, make sure they're remembered, close Dolphin
1. Open Firefox and download anything(the same page to html should be enough)
2a. Go to Firefox to open place where file was downloaded
2b. Click on the notification and choose Open Containing Folder

OBSERVED RESULT
Dolphin is opened with the right folder but previously remembered tabs disappeared.

EXPECTED RESULT
Dolphin is opened with all previously remembered tabs. If tab doesn't exist it opens new tab that leads to folder containing file.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro Linux
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 5.112.0
Qt Version: 5.15.11

ADDITIONAL INFORMATION
Wayland