Red Line on Invoices

I've been informed that some of my clients' invoices are only showing a diagonal red line, in both the PDF attachments and the 'view full invoice' clickable link. It's not for every invoice, just a handful over the past few months, so it seems like random chance. I've attached an example I was sent.

The solution used to be to change your logo size in your invoice template, but that doesn't seem to be an option anymore? I've also been using a lower res image, but that hasn't helped either. Any advice would be appreciated.