Your right it appears to be a Bug either brought on by the update to Yosemite, or the update to 14.4.5 Office a couple of days before which MS uploaded.
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Tell them exact as here, and if you can send a link to your screen shot. It will definitely put a crimp any one doing accounting because using numbers formatted -($123.55) is required format for CPA's, Government agency's, and Banks.
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