Up until (and including) version 12.17, my most crucial 3D image editing and display program opened my images on the "main" monitor (as specified in Win7 display settings), a 3D monitor.
I run my listers full-screen on the second, smaller (non 3D) monitor.
When I installed 12.18, the images now opened on the same monitor as the lister.
It's an inconvenience when I am editing, but a disaster when I am showing 3D images with the projectors connected. I need the image to open on the main screen!
I reverted to version 12.17: problem solved.
I don't know what you did in 12.18, but can you please consider undoing it, otherwise 12.17 will be my final version.
How are you opening the other program? Is it from Opus?
If it's from a file display (e.g. double-clicking a file), when you say it's full-screen do you mean maximized, or just resized to the edges of the screen? (There were changes in 12.18 that allow non-maximized windows to go closer to the screen edges, but they should not affect where other software opens its windows unless the other software is very strange.)
What happens when you do the same thing from File Explorer instead of Opus?
As Jon says above, Opus doesn't tell other software where to open its windows so if you're seeing a change between 12.17 and 12.18 it's very strange, unless we are missing a detail (e.g. if we're talking about using the Opus viewer to display the images or something).