I just opened it for the first time in a long time and went to the “Social” tab.
I don’t have a Social tab, but in Gmail’s Settings > Inbox Type, I’m using Priority Inbox. What inbox type are you using? Maybe that’s the difference?
It’s set to the Default Inbox, but also Settings / Inbox Customizations / Inbox Categories, I have them all turned on. As I previously said, I like the way Google does this. So, yes, I see the ads on the category lists sometimes - in this case, it was in the Social category. I just looked again, and I had some in the Promotions category.
See also Add or remove inbox categories in Gmail - Computer - Gmail Help
I also don’t like how the Gmail iOS app navigates among categories, which was the first reason why I stopped using a few years ago. (Then I switched my main email account to a different service.) iOS Mail in 18.2 allows for better navigation between categories.
So this is interesting—I did a switch to test and it does indeed look like the Default Inbox type gets ads, whereas the Priority Inbox type doesn’t. Who knew?