The source mapping for debuggers seems to do weird things, which breaks the breakpoint functionality in Qt Creator with lldb.
Reproduction steps:

• create a new C++ "Qt Console Application" project
• set a breakpoint in main.cpp
• run the debugger
• The debugger stops at the breakpoint, as expected
• Right Click Debugger > Configure Debugger > Source Paths Mapping
• Add a mapping from {/Users/somePathThatDoesNotExist}

  to the project folder (in my case its {/Users/sami/untitled}})

• Stop the debugger, and debug the project again

Expected outcome: the debugger stops at the breakpoint
Actual outcome: the debugger ignores the breakpoint (the breakpoint has a sandglass on it, to show that it wasn't loaded). Its a bit annoying because all breakpoints set via the QtCreator UI are not loaded.
(I'm using macOS)