`fill_form` with flatten option corrupts PDF file

[xenyal]

I'm trying to use pdftk's fill_form functionality with the below PDF along with the flatten option so that values populating text fields can be frozen. However, upon getting the output file, the PDF seems to become corrupted.

For additional context, I've embedded a TrueType font (Lato) into the Input PDF (using Apache PDFBox), and set the font_name on the appearance objects for all the text fields in the input document.

Input PDF: http://www.filedropper.com/prefilled

Output PDF (Cannot be opened): http://www.filedropper.com/output