There was indeed an issue where the DLL was not compiled with the necessary _ITERATOR_DEBUG_LEVEL
in order for the project to compile.
To the naked eye, it would seem that the DLL was compiled under the proper settings in debug mode. This was a lie.
Instead, in the Configuration Manager, the project was set to compile as release mode. So pressing the button to compile when it said "Debug" would actually compile in release instead. The issue gets chalked up as bad user interface design. It also means that the toolbar is unreliable.
TL;DR : Visual Studio's toolbar was a lie.