This has already been discussed somewhere here.
The “Extract Metadata for Mapping” button is not in sync with the internal variable. That means it may appear to be on, but it’s actually off. A simple workaround is to toggle the button off and on again.

@Christoph-Hart , I think this can be easily fixed in FileImportDialog on line 39:

metadataButton->setToggleState(true, sendNotification);

I figured it out.
In case someone else gets stuck at this point. When calling synchronously, the parameters of the function must be complete:

const var th = Engine.createTransportHandler();
th.setSyncMode(1);
th.setEnableGrid(true, 5);

inline function onGrid(grid, timestamp, first)  // Parameters need to be complete
{
    Console.print(grid);
}
th.setOnGridChange(true, onGrid);