The behavior "Under Mouse" for window placement works => as long as the active window is not on a different screen. If the active window is on a different screen it gets placed there instead.
Please provide output of xprop and xwininfo of a incorrectly placed window.
These would be empty on Wayland?
In future please include a hint that you are using Wayland.
Will do.
It does seem to affect all additional behaviours. Centered is affected as well.
I know maximised placement is broken with a top panel, I have a failing unit test ready. We get set the position, then handle a resize and end up going back to 0,0. I'll finish chasing that up, hopefully it's the same cause.
@Damian I got exactly the same behavior but on xorg, enabling "Active screen follows mouse" on Focus tab did fix it for me. (overall, <3 plasma)
>I'll finish chasing that up, hopefully it's the same cause. I fixed this. Please reopen if this original report is still valid with kwin > 5.16