The iPhone Mail.app was originally designed to work with a single account. So it had a set of preferences that made sense for a single account.
When Apple added support for multiple accounts, they did a pretty good job redoing Mail.app. Except, of course, for the preferences.
Even though I now get my business email and personal email on my iPhone, I have only one signature. Since the only thing that's common between my business and personal email is my name, there's not much use for that signature. I've removed it.
Since you don't see the signature when you compose a message, this is a bit of a subtle problem. It could even be embarrassing if, for example, your personal signature was a bit risque.--
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