I’m not getting an error on the hook. The hook is a workaround.
The error occurs on normal use.
The field group is defined in a class that is directly loaded from the functions.php. Usage is as in my example, define the fields then fetch the value after.
As mentioned in my initial post this only occurs for new options. The error is resolved after actually saving the option once on the settings page. This is because the database also holds a key -> name mapping resolving the issue.
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