Why doesn't GMail proxy some images through googleusercontent?

Question

gmail

I'm trying to add a 1px image to sent emails to track when a user opens emails.
Faced with an unpleasant fact - Gmail does not proxy some images through the googleusercontent server.
If you view the source of the email in Gmail, you can see that the image is there, but if you look in through the HTML inspector (f12 in Chrome), you can see that the images do not contain the src: tag. At the same time, an image from another host lies nearby and it is normally proxied through googleusercontent.
I have tried not only single pixel images but also normal images from other hosts.
What could be the problem?