After being logged out by the app, some have found that they receive a “Welcome to Instagram” message when they log back into their account, which suggests that Instagram is behaving as if they are a new user.
The issue seems to be connected to the broader problems that Instagram is currently working through, and will hopefully be resolved when the app is back to proper working order.
Source: https://www.distractify.com/p/why-is-my-instagram-saying-welcome-to-instagram