Maven Resolver (at least the 3.6.0 one) model processor uses ReaderFactory#newXmlReader to create a XmlStreamReader and then pass it to MXParser.

Problem is that since 1e18ddc MXParser assumes the input encoding is UTF8 and only changes that if the input reader is a InputStreamReader (which is not the case here since it's a XmlStreamReader). So we end up with exception:

UTF-8 BOM plus xml decl of ISO-8859-1 is incompatible

This whole check is not really needed here since XmlStreamReader is taking care of potential encoding issues already from what I understand.