configSource valid relative path in app.config throws System.Configuration.ConfigurationErrorsException: 'The configSource attribute must be a relative physical path. (C:\Projekte\MyProject\bin\Debug\.build\Debug\net6.0-windows\MyProject.dll.config line 7)'

[wederw]

In a .NET Framework 4.8 project i used the configSource in the App.config to load a custom configuration like following:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="customConfig" type="MyProject.CustomConfigSection, MyProject.Backend" />
  </configSections>

  <customConfig configSource=".\Custom\Config.config" />

</configuration>

Now i try to to do the same in a .NET 6.0 project, but i throws the exception System.Configuration.ConfigurationErrorsException: 'The configSource attribute must be a relative physical path. (C:\Projekte\MyProject\bin\Debug\.build\Debug\net6.0-windows\MyProject.dll.config line 7)' in System.Configuration.BaseConfigurationRecord.NormalizeConfigSource @3206 because the System.Configuration.ConfigPathUtility.IsValid returns false @45. I can modify the configSource in a way that IsValid returns true, but then the xmlreader can't find the Config.config. Can someone explain me what i'm doing wrong or is this a bug?