If you use the scale function from Windows OS (System>Display>Scale) and select a value different from 100%, in a .net fwk 4.7.2 application, when opening a modal window/form the Applications font scales down to the default (100% windows scale). I mention that the child form has the AutoScaleMode property set to Font. You can observe the same manifestation when attaching to the application for debugging using Visual Studio (giving that you have the scale changed on your machine).
I tried with a blank .net fwk app and I didn't managed to reproduce the same outcome. I think that the problem of the application in with the problem occurs is the contents of the child form that i want to open. There is a wpf window that is rendered inside of that control.