Hi bobi_frankey,
From the description you provided, it sounds like you may be working in multiple spaces. Work in multiple spaces on Mac describes how that can be configured, as well as offers instructions to assign apps to specific spaces:
"Assign apps to spaces
If you use two or more spaces, you can assign an app (or System Settings) to a specific space so it always opens in that space.
- On your Mac, Control-click an app’s icon in the Dock.
- You may need to first open the app to show its icon in the Dock.
- In the shortcut menu that appears, choose Options, then choose one of the following:
- All Desktops: The app opens in every space.
- This Desktop: The app opens only in the current space. If you use the app full screen, it appears in its own space.
- Desktop on Display [number]: The app opens in the current space on a specific display (if more than one display is available).
- None: The app opens in whichever space you’re using at the time."
By default, when switching to an app, the desktop automatically switches to a space that has open windows for the app. For example, if you create a new TextEdit document in Desktop 3, but TextEdit windows are already open in Desktop 2, your new document opens in Desktop 2. To change this setting, choose Apple menu 
> System Settings, click Desktop & Dock 
in the sidebar, go to Mission Control on the right, then turn “When switching to an application, switch to a Space with open windows for the application” on or off."
Try checking those settings to see if this has been configured and to adjust the settings if so to stop that unwanted behavior.
Regards.
