If the issue doesn't appear in non-Qt apps, then some setting in this particular application could be compiling with optimizations turned on.
It gets confusing in Visual Studio, but nothing stops a "Debug" build to have optimizations turned on, and a "Release" build to have optimizations turned off (more often, you will see Release builds with optimizations turned off).
On top of that, it is possible to have a "Debug" build that cannot be debugged due to the debug symbols not being produced (two more settings in the Project turns on/off symbols during compilation and linking). In other words, any combination of "debugging on/off" and "optimizations on/off" is possible in "Release" and "Debug" build configurations.
Since you are producing symbols, the issue won't be that the symbols aren't being produced. Therefore, please check all of the projects you're building, and look at the "Optimizations" setting, and not whether the configuration is named "Debug" or "Release".