Field missing in PDF if another field on same line is empty

Hi, sometimes when the PDF is generated one or more of the fields are missing from the output. It seems to be related to whether another optional field set to appear on the same line of the PDF is empty or not. If it is empty, neither of the fields appear in the PDF even though the field with data appears in the CF7 email.
This worked fine for a year and has only recently started happening although no changes have been made to those fields in either the contact form or the PDF code.
Thanks for your support.

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