Is encrypting PII on the server mandatory for App Store compliance?

I’m building an iOS app that collects user PII (emails, names) and stores it in my backend database. I already use HTTPS for data transfer, but I’m unsure if Apple requires server-side encryption for stored data. For example: If a user’s email is stored in plain text on my server (but transmitted securely via HTTPS), will this violate App Store guidelines? Does Apple explicitly mandate encryption-at-rest for PII, or is it just a recommendation? Are there exceptions for non-sensitive data like usernames? I checked App Store Review Guidelines 5.1.1, which says "data must be stored securely," but...