I’m having the same problem, my tracking stopped shortly after 8am on Sept 15, 2020. I sent a test email this evening to a gmail account and opened it on a mobile device that downloaded the images in my signature (so presumably the pixel image downloaded too) but it’s still not showing “opened” in my gmail inbox. I only send from gmail and use the sales extension & it’s been working the last several weeks just find until 3 days ago.