That's probably because it's full of bot followers which isn't an organic kind of Instagram account so it's not going to garner you real interaction. They do this kind of thing where they grow an instagram account and build a bot following that's going to make it gain some real followers albeit not enough to make organic engagements. And once it's out of their hands, it's going to be a useless account with thousands of followers on the people that's bought the sold account.
I'll say, it's too late now as the account has been sold and the buyer seems not to have made any complaints bout it, even though its too early to spot out whether the engagement is low or not, bots or not but let's hope its not as you described anyways.
