I've found a workaround about this problem. The problem was because, the watch app target wasn't added to a main project. So, the target was there, and i can build for ios or watch os, but they weren't joined together. When project consists only of both (ios and watchos) apps on swiftUI (without any storyboard), somehow, the xcode builds them all together, and app doesn't disappear from the watch after a wile. But in case, where ios part is on storyboard, the swiftui watchkit will not be connected to the ios app as it should be. In order to join them together, there are next steps needed (i've found it from one article. here's a text from this article):
"Add the watch target to the main application
In the project navigator, select the target for your main application.
In the main pane, go to the General tab.
Scroll down to the Frameworks, Libraries, and Embedded Content section.
Click the + button and choose the watch app target from the list.
The watch app target should now be added to the list.
By adding the watch app target to the main application, you are indicating that the main application depends on the watch app and that the watch app should be included in the final app bundle when you build and run the main application. This is necessary in order for the watch app to be installed and run on the Apple Watch when the main application is installed on the iOS device."