I tested the new variable font support in 6.7.0 Beta 2, and while it works wonderfully (thank you, by the way), features set on a QFont instance in C++ code during startup seems to be set on unrelated fonts in QtQuick.

When my application starts, I load fonts into QFontDatabase, get a QFont instance for my default preferred font, and set features on it:

This works as expected, but then I load additional fonts for the application, the same way with QFontDatabase. I don't set any features on them. However, when looking at the UI, I can see that, for example, the "ss01" feature is applied to another font family. 

The family I set the features on is called "InterVariable", these headers use the family "Torus":

With no features set on InterVariable:

With features set on InterVariable:

This is a regular QtQuick label, with font.features not set on it. When I manually specify font.features: { "ss01": 0 }, the issue goes away. I would expect it to have that default value though.

Just tested, getting a QFont instance from the supposed to be unaffected Torus font on the C++ side, and setting "ss01" to 0 does not affect the font in Quick, just like I would expect. Maybe only the application default font's features affect the others? If that is the case, I would still consider this as a bug, since setting features on a specific QFont object shouldn't affect unrelated QFont objects, even if the one I set the feature on is the application default.