Mix up your hashtag game would by my first go-to.
Play around with new hashtags, cutting out irrelevant / potentially shadow-ban inducing ones and try different amounts of tags per post to see how they influence engagement.
Also if you're using a bot to make actions / post it might be worth seeing how your engagement fares going 'legit' with no automation and posting through the app.
If this like drop is quite recent though, ie. since Instagram has started removing likes from some accounts: plenty of people have seen fairly dramatic engagement drops - it could easily be related to this.