Specific app selection not possible...if with the same name!

Good morning,
Your issue revolves around Windows associating two different file types (.zap17 and .zap18) with the same application, irrespective of version. What Windows is doing is associating these files by the application's executable name (in this case, Siemens.Automation.Portal.exe).
Unfortunately, Windows doesn't handle versions separation quite well. But there are some workarounds, one of which involves editing the Windows Registry; however, this method is generally not recommended if you're not comfortable with modifying system settings.
A safer, alternative method is to use a third-party app, such as “Open with++”.
"Open with++" is a shell extension that provides detailed options for file execution, this can be put to use to execute a particular type of file with a specified version of your application. It could allow you to open .zap17 files with TiaPortalV17 and .zap18 with TiaPortalV18. Here are the steps:
1. Download and install "Open with++" from its GitHub repository.
2. After installation, right-click on any .zap17 file.
3. Go to the "Open with++" option, then click on "Edit Commands".
4. In the editor, add the full path to your TiaPortalV17 app's Siemens.Automation.Portal.exe [like C:\Program Files\Siemens\Automation\Portal V17\Bin\Siemens.Automation.Portal.exe (replace with the actual path to your .exe)].
5. Save changes
Repeat the process for .zap18 files, but this time, you'll target the TiaPortalV18 version. This way, each of the file types has its own unique application link which should help to overcome the issue.
Please remember to create a system restore point before making any changes to your system! Changes can have unforeseen effects and potentially impact system performance or stability.