It's essentially the same bug. If someone has long-lasting tell/who bug that persists through zoning and relogs, it's because they have another client logged into the account from the same IP. The system registers the character as being offline because of the second client, so you can't send them tells or see them on zone /who lists.
I'm not 100% sure what causes the temporary tell/who bug, but I usually see it happen to people who are trying to switch characters quickly. Waiting a few minutes or zoning always fixes this. If it doesn't go away, it's because of an open client.
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