@HumanNo1 I can confirm this occurring on openSUSE Tumbleweed, using KDE Plasma 6. Installing Chromium and leaving it, Vivaldi and Firefox open, then logging out and logging back in, only Firefox launched automatically.
Since Chromium also did not automatically relaunch, I suspect this is upstream with Chromium and not an actual bug in Vivaldi, but it's quite possible that this behavior is by design.
In any event, I submitted a report (VB-108269) and referenced this thread.