Just tried using the Pre-packaged .NET Framework 4.8 installer with build 1909 of Windows. It looks like that .NET framework installer is setup to look at "HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\Release" with a version comparison greater than 528048. However, on Windows 10 1909, the installer reports that .NET 4.8 is already instead. When I look in the registry, it appears that the Release value is 528040. There's obviously some discrepancy between what Windows includes in 1909, and what the .NET 4.8 installer actually deploys, but it's making it look like .NET 4.8 always needs to be installed on Windows 10 1909.
Indeed, you are right. This prerequisite actually compares two values, based on Windows version, as it follows:
1. .NET Framework 4.8 installed on Windows 10 May 2019 Update 528040
2. .NET Framework 4.8 installed on all OS versions other than Windows 10 May 2019 Update 528049
To fix this, you can copy the condition from the "Prerequisites" page --> "Install Conditions" tab and change the "Value" field from 528048 to 528039.
Also, please make sure that the "Install prerequisite if all conditions are false" option is selected.