Thanks Wolfgang for explanation, as always very clear.

Another related question:
If I want to use italic numbers in math, I use:
\definefallbackfamily[...][math][...][tf=style:italic,preset=math:digitsnormal]

But I lose upright numbers this way.

How can I solve this issue. I've tried without success:
\definefallbackfamily[...][math][...][preset=math:digitsitalic]