Error during CMake using qtquick_compiler_add_resources or QT5_ADD_RESOURCES

CMake Error at C:/Qt/5.12.7/mingw73_32/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:211 (configure_file):
 configure_file Problem configuring file
 Call Stack (most recent call first):
 C:/Qt/5.12.7/mingw73_32/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:281 (_QT5_PARSE_QRC_FILE)
 C:/Qt/5.12.7/mingw73_32/lib/cmake/Qt5QuickCompiler/Qt5QuickCompilerConfig.cmake:84 (qt5_add_resources)
 mtest/CMakeLists.txt:38 (qtquick_compiler_add_resources)

Here with MinGW 32-bit, but similar with MSVC and 64-bit.

The very same setup (of CMakeLists.txt etc.) does work just fine in Qt 5.12.6 and even in the Qt 5.15.0 snapshot, so to me looks like a regresssion in the Qt 5.12.7 snapshot. So far the only thing I've changed in our (MuseScore's) CMakeLists.txt to take advantage of the newer (since Qt 5.11) qtquick_compiler_add_resources (from formerly using QT5_ADD_RESOURCES) in https://github.com/musescore/MuseScore/pull/5649

Those changes  don't seem relevant to the issue, I got the same (or very similar) errors before, but thought those changes might fix them (and would be a good thing anyhow). 

Baseline: with or without those changes I can't get our project  (MuseScore) to build using Qt 5.12.7 snapshot, but can with all (well, some) other versions.

I guess that makes it a Blocker issue, doesn't it?