Contact list is a note of kind=3 and should always contain the full list. When you add a contact or remove, your client produces a new kind=3 note that supersedes the previous. https://github.com/nostr-protocol/nips/blob/master/02.md

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It could be that your relays dropped your list, or maybe somehow your client switched relays (or you’re using a different client which talks to different relays).