Windeployqt fails with "Unable to find the platform plugin" when using --release switch

See also: https://bugreports.qt.io/browse/QTBUG-80806

Steps to replicate:

Qt 5.15.0 installed with only MinGW 8.1.0 x64 via the online installer.

Create a new project via Qt Creator: Application (Qt) > Qt Widgets Application

Accept all defaults for the project (Desktop Qt 5.15.0 MinGW 64-bit kit)

Change build version to Release and Run

Open the Qt 5.15.0 command prompt from the Start menu

Note: Another bug - MinGW version is actually 8.1.0 but all the shortcuts and window title incorrectly state MinGW 7.3.0. (version verified below).

C:\Test>windeployqt --release "C:\Test"
C:\Test\untitled.exe 64 bit, release executable
Direct dependencies: Qt5Core Qt5Widgets
All dependencies : Qt5Core Qt5Gui Qt5Widgets
To be deployed : Qt5Core Qt5Gui Qt5Widgets
Unable to find the platform plugin.

 
MinGW version:

C:\Test>gcc --version
gcc (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0