I’m not sure if I could pinpoint one exact individual to start this. But I’d say as early as 18th century? The fact that there wasn’t an equivalent translation to what the glyphs meant at the time sort of gave the historians an idea of the ‘mystical’ nature of the images, being so complex and so few they have to represent an entire language. They knew the later stage, Coptic, but that was because it largely utilized Greek letters and was still being used by Coptic Christians in religious settings. The idea of hieroglyphs as complex isn’t wrong because it is in its own way, but the scale might have been exaggerated especially when the decipherment period was also the Egyptomania period where Victorian era Europe sort of fetishized Egyptian culture. A reason it’s still around is that it never truly left that pop culture spot because Pop culture sort of reuses the same Victorian themes that Egyptians were obsessed with death and curses (the reason that the two biggest icons representing egypt are pharaohs and mummies) and most archaeologists and Egyptologists are working to undo that damage. Hope that helps a little!