This line: https://github.com/DocnetUK/tfpdf/blob/master/src/tFPDF/PDF.php#L1033
will include the font definition file which is previously saved with the variable $desc holding the font descriptor keys and values (here). The $desc variable is not used further but the font descriptor is taken from the empty array: https://github.com/DocnetUK/tfpdf/blob/master/src/tFPDF/PDF.php#L1035 then.
This results in corrupted PDF documents.