It's nothing to do with Windows 10. It's up to each individual application where it opens its windows, and how/if it saves previous positions.
Opus defaults to opening new windows where the mouse cursor is, but you can change that.
For the default lister, there are three options under Preferences / Launching Opus / Default Lister:
Choose Always in the same position if you want it to be where you saved it, and use Settings > Set as Default Lister in the lister itself to save that as your preferred position.
If you're opening a Layout rather than the default lister, then there's a setting in each layout which controls where it opens (you can set this when saving a new layout, as well as when editing existing layouts in Preferences):
If Open layout relative to the monitor the mouse is currently on is turned off, then the layout will open the windows exactly where they were when you saved them.
(In both cases, this assumes the monitors and screen positions still exist. If you save a layout with a window on a second monitor which is then unplugged, the window will be moved to another monitor when you open it, since it would be invisible otherwise.)
(This is also assuming that ZMover or a similar tool is not moving other application's windows after they open. If you're seeing everything not remember positions where things used to, I'd suspect something like that was involved, rather than an OS change.)