I think with instagram there’s something a lot of people don’t understand with using tags. I don’t know if you’re using them but I guess this is just something I’ll talk about in general because I see conversations about tags all the time.
A lot of the people who use tags and engage with tags are bots and this means a lot of different things that I don’t think people really know about. There are people who use bots for their art accounts to “like” everything in that tag to get more traffic to their own art/comic pages. There are also bots that allow people to automatically follow people posting in those tags and then automatically unfollow after a certain amount of time.
And I’ve KNOWN people who have experimented with those methods so I know it’s not even uncommon. It’s one of the reasons I’ve never used tags because it’s just NOT a reliable way to get real engagement and the numbers you get might not end up meaning anything at all.
That said, the people I’ve known who have tried this generally stop because it’s not a good long term strategy for making actual engaged readers. But it’s very possible the person you’re talking about was just using bots and was never even aware they were following you.