In UTF-8 “0xC5 0x93 (c593)” corresponds to http://www.fileformat.info/info/unicode/char/c593/index.htm
which is definitely not correct.

> My understanding is that the barcode string our example is UTF-8 encoded.

The simplest test to ensure your doc is wrongly encoded would be to open the document in a web browser. After that change the encoding to windows-1252 and open the doc in another tab. Compare and you’ll see the barcode appears slightly different (prefixed and postfixed in the second case). In the first case the start-stop chars are not displayed (until CJK fonts pack installed).

> the Unicode character 0x0153 (the barcode STOP character)

It is not correct. 153 is not hex, but decimal ASCII value (= 0x99), which is in UTF-8 should be encoded as 0x2122