There’s a long-standing issue where scale factors on Windows are reported differently depending on whether the project is running as a plugin/library or a standalone app. Although this is something I’d like to address, it’s likely to be disruptive to existing projects, and our focus is on other issues at the moment.
Opening additional windows in a plugin is generally not a good idea. Apple even explicitly recommends against opening new floating windows in plugins in their official documentation.
I understand that some designs require additional top-level windows; however, I’d recommend looking for alternatives wherever possible.