I have two monitors with the Primary on the right.
After trying all suggestions the only thing that worked for me was "Uncheck (Allow apps to remember the positions of their own windows, if they support it) under System Settings → Window Management → Window Behavior → Advanced.
Everything else was left as standard out of the box fresh install of latest Kubuntu with all updates.

I wrote a set of KWin scripts to solve this problem:
- always open on active screen (the one with the mouse cursor): https://store.kde.org/p/1617640/
- always open on focused screen (the one with the focused window): https://store.kde.org/p/1618008/
- always open on primary screen (the one set in the display settings): https://store.kde.org/p/1617641/
Hope this fixes it for you.