I'm not 100% sure, but it looks like Conpherence posts sent via email as replies of notifications will include the quoted text below the reply, where Maniphest tasks know how to filter it.
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We should be running the same code on both, so this might be a specific mail client's quoting conventions, as @chad suggests. There's no standard way for us to strip off quoted text or signatures from email so we kind of do a collection of things that usually work and just add more rules as we run into cases we don't handle correctly.
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Hm, I tested here and it worked well. Nevermind, then. I'm closing this task as Invalid and I will reopen if I can reproduce.