Hello team, I'm a bit confused on how I should handle this: Users of my app will start anonymous, and should get a feature flag immediately to determine which onboarding they go through. However, in the middle of onboarding, they will log in, which I would probably set a Growthbook attribute for their user id. But how can I ensure the feature flags don't change once they log in? Won't it change, since i just added a growthbook attribute? Maybe I should instead save the user's device ID as the attribute?

Is the onboarding flow set up as an A/B test? If so, you can use sticky bucketing with a fallback attribute: user id (primary) + anonymous id (fallback). This will ensure user bucketing is persisted beyond the login.