Attached you find a simple QML test program rendering QML Text elements.

If you run it on a high DPI system with QT 5.12.4 kits msvc2017_64 and winrt_x64_msvc2017, you see that the screens are almost identical (screenshots attached). If you do the same with 5.12.5, the WinRT version shows that the dimensions of QML Text are too small and the window has a different pixel count with the same physical screen size (screenshots attached).
Investigating the souce code, I have found the commit that causes this: 589d96b9 "winrt: Remove QWinrtScreen::pixelDensity so that the default of 1.0 is used".