When working and viewing in adobe express, my text formatting looks correct, with part of the last sentence in the large paragraph extra bold italic. When I download it as a pdf, it makes the entire last sentence extra bold italic.
I had a problem with something similar. The font I chose in Express (Futura PT medium oblique) changed to Futura PT book when I downloaded my project to PDF. The .png and .jpg downloads looked fine; it was the PDF I was having a problem with. I had to do everything but stand on my head to make it finally work. It might have been one of these steps that did the trick. Here is what I did:
Copied the entire file to a new file
Went to Adobe Fonts in Adobe Creative Cloud and added the font family of the font I was after
Opened a Notepad file and pasted my copy into that to "clean" it
Created new text boxes in my project to paste the "cleaned" copy into it
Changed the font to the desired font in my newly added font family
After doing all of this, I finally got a decent PDF with the fonts I wanted.