I'm bumping this thread to report a bit of new info. I had this issue in two different machines. One I tried to format and reinstall windows and the issue went away So far that's the ONLY way I've found to deal with this persistent "illness". On the other one (which is the one I don't want to format) the issue still remains, even after the latest 1803 update. I also tried the "in-place upgrade" "windows 10 refresh" trick, which worked fine but again after the first Windows restart, again the same issue. I'm now positive that what is going on is that the app in question is being blocked access to a specific font that it was designed to work with and it reverts to another font which looks like "small and blurry". So I think it's not a font size changing issue but that it switches to a totally different (and inappropriate) font. My god, this is a nightmare. Something probably in the registry is stuck and keeps doing something with the system fonts no matter if a whole Windows build update takes place! I exported the registry before restarting and the size was 332MB. After the restart I did again and this time was 288MB. Perhaps this is natural, just saying... In desperation. To format this machine means two days of painful work, has lots of installations and is in a work environment. So any ideas would be really appreciated.
Just wanted to report that finally I found a workaround! I deleted all the entries of [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes] and that fixed it! I know it is not the most elegant way but it works. After that the app in question does not use a substitute font as it was doing before and uses the correct font instead. The entries were the same as in another machine that was "healthy" so the issue is not exactly this but something else that triggers this font substitution to take place on the "ill" machine. I don't know what that is or whether I'm ever going to find out. For now case is solved.