I was trying to patch my Windows Server 2012s through a patching service, however the 4.5.2 patch was not showing up as applicable. After some digging with the service, we figured out that the registry key was indicating we only had 4.5 installed. However, I can manually patch the 4.5.2 framework just fine with a standalone installer. Shouldn't Windows Updates find this not applicable or why would this be?
The Windows Update detection logic is correct. .NET 4.5 is no longer supported, so it should be expected there would be no offering. You may want to consider updating to a version that is not end of life.
4.5.2 is technically a different version of .Net than 4.5 and not just a patch. So it would be correct to not be offered a patch for 4.5.2 when 4.5 is installed but still be able to update to 4.5.2 with a 4.5.2 installer.
As @greg askew has said however, 4.5 is no longer supported. I would recommend updating to 4.8 which is the current version. See https://docs.microsoft.com/en-us/dotnet/framework/migration-guide/versions-and-dependencies for the available versions and the detection registry keys.