I have come across this before. When you reduce, you need to specify the encoding. In the configuration of your project for xml encoding, specify:
<?xml version="1.0" encoding="utf-8"?>
If you are uncomfortable with the configuration or have no access, you can change the CSS.
@charset "UTF-8";
In both cases, this should be the first line of the file. If you are concatenating, it should only be in the first file.
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