qmake always wants to add NDEBUG to the DEFINES var. With another version (e.g. 5.12.6) this does not happen.

This can be tested by opening a "Native Tools Command Prompt for VS" and pasting the following:

ECHO CONFIG -= debug release debug_and_release > foo.pro&
ECHO CONFIG += (Config) no_fixpath >> foo.pro&
ECHO QT += core >> foo.pro&
(Path to Qt)\bin\qmake&
TYPE Makefile | FINDSTR /B "DEFINES" 2> NUL&
DEL foo.pro Makefile

E.g.:

5.13.2 - Debug

• Command:

  ECHO CONFIG -= debug release debug_and_release > foo.pro&
  ECHO CONFIG += debug no_fixpath >> foo.pro&
  ECHO QT += core >> foo.pro&
  \qt\5.13.2\msvc2017_64\bin\qmake&
  TYPE Makefile | FINDSTR /B "DEFINES" 2> NUL&
  DEL foo.pro Makefile

• Output:

  DEFINES = -DUNICODE -D_UNICODE -DWIN32 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -DWIN64 -DNDEBUG -DQT_GUI_LIB -DQT_CORE_LIB

5.13.2 - Release

• Command:

  ECHO CONFIG -= debug release debug_and_release > foo.pro&
  ECHO CONFIG += release no_fixpath >> foo.pro&
  ECHO QT += core >> foo.pro&
  \qt\5.13.2\msvc2017_64\bin\qmake&
  TYPE Makefile | FINDSTR /B "DEFINES" 2> NUL&
  DEL foo.pro Makefile

• Output:

  DEFINES = -DUNICODE -D_UNICODE -DWIN32 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -DWIN64 -DNDEBUG -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB

5.12.6 - Debug

• Command:

  ECHO CONFIG -= debug release debug_and_release > foo.pro&
  ECHO CONFIG += debug no_fixpath >> foo.pro&
  ECHO QT += core >> foo.pro&
  \qt\5.12.6\msvc2017_64\bin\qmake&
  TYPE Makefile | FINDSTR /B "DEFINES" 2> NUL&
  DEL foo.pro Makefile

• Output:

  DEFINES = -DUNICODE -D_UNICODE -DWIN32 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -DWIN64 -DQT_GUI_LIB -DQT_CORE_LIB

5.12.6 - Release

• Command:

  ECHO CONFIG -= debug release debug_and_release > foo.pro&
  ECHO CONFIG += release no_fixpath >> foo.pro&
  ECHO QT += core >> foo.pro&
  \qt\5.12.6\msvc2017_64\bin\qmake&
  TYPE Makefile | FINDSTR /B "DEFINES" 2> NUL&
  DEL foo.pro Makefile

• Output:

  DEFINES = -DUNICODE -D_UNICODE -DWIN32 -D_ENABLE_EXTENDED_ALIGNED_STORAGE -DWIN64 -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DNDEBUG