I worked for someone back around 2013/14 in which we tried to implement some tracking pixels, and what you described was exactly what we ran into with Gmail rec

I worked for someone back around 2013/14 in which we tried to implement some tracking pixels, and what you described was exactly what we ran into with Gmail recipients. When the e-mail lands in the inbox, Google was retrieving the image from your server, caching it, and then displaying the cached version to the user -- so all you knew was that the email was being delivered to the inbox, but couldn't tell if the email was opened. I don't know if that's how it currently works, but that's what we ran into at the time IIRC.