Toolbar contents are not vertically centered with Qt 5.13 anymore. That may not be easily seen with Fusion but, in the attached screenshot, I've intentionally set "PM_ToolBarItemMargin" and "PM_ToolBarFrameWidth" to 10 for it to be clearly visible with the Kvantum widget style.
I've also attached a diff file that shows the differences between qtoolbarlayout.cpp in Qt 5.12 and Qt 5.13. The coefficient 2 is missing in several places of Qt 5.13 (search for "2*" in the diff file).
EDIT: The fix is also attached.