When debugging is started, it is sufficient to put a breakpoint anywhere and the running application instantaneously closes. That breakpoint can be set in any document open in Creator, even in code that is totally independent from the running application. Application Output panel in QtCreator simply states "Debugging has finished". This happens only with breakpoints (and message tracepoints) added on-the-run, setting them before starting debugger does not result in problems.

This also happens when using older Qt versions (I tested only 5.9.2). On the contrary using Qt 5.10.1 through QtCreator 4.5.0 works fine. These things combined point out that the problem is with Creator 4.5.1.