That's probably the DPI scaling. If you've scaled the text to (say) 125%, any apps that support scaling will simply scale the font by 125%. But on apps which don't support scaling, Windows makes a copy of the app window and scales that 125%. The result is that text appears blurry.
You can "fix" it by turning off DPI scaling for that app, but doing so will make the text appear smaller. Probably won't be a problem on a 15.6" screen. Right-click the app in question, and open its properties. Go to the Compatibility tab. Towards the bottom there should be an option to "Disable display scaling on high DPI settings". Check it, and click OK.