Thank you, @Dan C. I'm not clear how you found it, but I'm glad you did. Much appreciated. I'm going to go with the theory that it can from pasting in styled text, as I can't otherwise understand how happened. Thanks again.
Thanks for your document There appears to be a change of font after a hard return in one of your text boxes, the font is only changed for this return character and as it's otherwise an empty line, so it's certainly easy to miss!
Please see my below screen recording showing where this is hiding, I have done my best to blur any parts of your document I don't need to show and confirmed that after removing this line and saving the document, I no longer see the missing font warning when opening your document.
2020-05-04 08-48-46.mp4
I hope this helps!
Gordon Meyer got a reaction from Dan C in Designer: Missing Font warning