One case in point is Skype for Business. For some reason it scales correctly as I move between the Surface screen and the external monitor when I use the Dell, but on the LG monitor Skype is either massive on the external monitor, or tiny on the Surface screen.
After a big of digging around I came across a solution that worked for me, which is to change a setting in Skype's manifest file (who knew there was one?). On my machine the file is here:
C:\Program Files\Microsoft Office\Office16\LYNC.EXE.MANIFEST
And the setting in question is this:
Which I changed to this:
Note that you'll probably have to edit the file as administrator in order to be able to save it.
Once I made that change, saved the file, and restarted Skype, it now scales correctly for both the Surface screen and the external LG monitor.