Do you mean besides Facebook’s own internal research that shows it’s harmful? It seems like their employees would know best, since they have access to the most data.
https://www.wsj.com/amp/articles/facebook-knows-instagram-is...
https://www.wsj.com/amp/articles/facebook-knows-instagram-is...
More direct to your question: Not necessarily anything more than their research, but something that details what specifically their research shows and what those research methods were?
FWIW: I did end up finding a nonpaywall version, and the research wasn't super reliable / great. They asked people specifically how instagram made them feel, which is subject to a lot of biases and not a solid objective indicator. I understand it may be the best we do, but that's not a great reason to trust it.