Flat Style look on deployed Qt application

I have an application which correctly displays the WindowsVista style on my development machine. If I go to the output directory, and run windeployqt, the WindowsVista style is no longer available. If I delete Qt5Core.dll from the output directory, then WindowsVista style is again available. (video attached of this)

Upon installing that application onto my test machine, including the required Qt DLLs, the WindowsVista style is not available; only Windows and Fusion. If I install QT5.10 on the test machine, the WindowsVista style becomes available again.

This issue only began after upgrading to Qt 5.10, no problems with 5.6 or 5.9.2