While QTBUG 80763 fixes windeployqt's problems for MinGW Qt 5.14.0 for debug builds, the bug still exists for release builds.
The reason is that the flag NT FileHeader flag "Debug information stripped" is no longer present in release build of qwindows.dll. So the logic in determineDebugAndDependentLibs() in utils.cpp incorrectly thinks that all .dlls are debug flavored, line 828 and 829:
// Use logic that's used e.g. in objdump / pfd library
*isDebugIn = !(nth->FileHeader.Characteristics & IMAGE_FILE_DEBUG_STRIPPED);
I currently use the workaround
*isDebugIn = false;
since I never use debug builds, but it's a bit crude