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?