not sure if it makes a difference but Im running this on a Surface Pro
Pretty much with any modern app that opens a dialog box. Similar to the right click problem - for example running this in modern IE on the desktop and when I right click on the circle with the 3 dots to open a new in private tab, the drop down appears very briefly before disappearing. However, in the case of the dialog boxes, it just goes into the background until I close all the other apps running on the desktop then it reappears and functions normally.
Seems like when a more than one modern app is open on the desktop, if the app brings up a dialog box, the dialog box appears but when something is clicked in it rather than responding to the click as it should it just disappears into the background. Either closing all the other apps on the desktop or making the modern app go fullscreen outside the desktop seems to correct this.