A ZNC server is often used with several clients in a parallel way all using the same ZNC nick. All these clients also have the same IRC nick, e.g. the nick which is visible to other participants of the IRC server.
Messages sent be an IRC nick, however, are not decrypted by ircrypt-hexchat. This means you can not read your own messages with HexChat/ircypt-hexchat that have been sent with another client (and of course the same IRC nick).