The sender is only displayed as "" only in test messages, as test messages never route through Seller Central's messaging system. Since the test messages are just for local testing, and using your email address, it may show the name configured for that email address directly or the business name as entered at sign up. This business name may be the same or different from your actual Seller Name on Amazon (as seen on your Seller Profile). This would not be an issue with Active messages since they are not sent directly through the test message email address.
Active mode messages originate from Feedback Genius, route through Seller Central, then are sent from Amazon's email system using the special email address that you see for the buyers (they look like Amazon's system will then put your Seller name as it appears on Amazon, and a similar, cryptic-looking email address in the "From" address, and send that to the buyer.

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