When you're designing an API, you're bound to run into messy user data. So, how do you deal with it?

I've been running into some interesting challenges with user data across different services. We've got user profiles, authentication details, and billing information, all managed separately. Sometimes, updates don't propagate perfectly, leading to temporary inconsistencies. For example, a user might update their email in one place, but it takes a bit for it to reflect everywhere else. What are your go-to strategies or patterns for managing this kind of eventual consistency gracefully in an API-first design? Any specific architectural choices or libraries you've found helpful?