Security 101 when changing the email of an account for any reason: email the old account and let it know the change happened. The weird thing is I know the Instagram security team, and they are top notch. I have a feeling this was vibe coded by someone outside of security and security wasn't looped in.
The fact that this can happen at all without the security team's knowledge is telling.
The security team at any organization is always considered an enemy to product and innovation. It wouldn't be surprising if management made it impossible for them to put in place the monitoring necessary to know this was happening. Especially at somewhere whose motto is "move fast and break things".