Some background first: the UTF-8 BOM is a sequence of bytes (EF BB BF) that allows the reader to identify the file as an UTF-8 file.
Normally, the BOM is used to signal the endianness of the encoding, but since endianness is irrelevant to UTF-8, the BOM is unnecessary.
According to the Unicode standard, the BOM for UTF-8 files is not recommended:
2.6 Encoding Schemes ... Use of a BOM is neither required nor recommended for UTF-8, but may be encountered in contexts where UTF-8 data is converted from other encoding forms that use a BOM or where the BOM is used as a UTF-8 signature. See the “Byte Order Mark” subsection in Section 16.8, Specials, for more information.
If you have troubles with this source, you can add a CHARSET to the props.conf on the input of this source.
Example: if you have a universal forwarder, add it into props.conf of the universal forwarder to set a CHARSET.
hope this helps ...