It is actually checked and only the appearance is wrong. When the file is saved the checkboxes states are as expected.

This behavior is observed with PdfImportSettings.ReadOnDemand setting only.

When making changes to Form fields then flattening them and invoking the viewer`s Save method an exception is thrown: System.Collections.Generic.KeyNotFoundException: 'The given key was not present in the dictionary.'

Code to reproduce the issue:

this.pdfViewer.Document.AcroForm.FlattenFormFields();

using (Stream output = File.OpenWrite("Exported.pdf"))
{
	this.pdfViewer.Save(output);
}

this.pdfViewer.Document.AcroForm.FlattenFormFields();

using (Stream output = File.OpenWrite("Exported.pdf"))
{
	PdfFormatProvider provider = new PdfFormatProvider();
	provider.Export(this.pdfViewer.Document, output);
}

The thinnest line thickness is thinner than in Adobe and it is not scaled according to the scale factor.

Observed: