Unable to validate XML against XSD schema

Question

I am working on a Python project where we have a function generating an XML Schema from data gathered from different sources. The main objective is not to validate the output of this function using an authoritative schema (namely https://github.com/ietf-tools/xml2rfc/blob/main/xml2rfc/data/v3.rnc).

The official authoritative schema is in Relax NG Compact (RNC) format. Thus, I converted this schema into an XSD schema using the trang official converter tool.

Here is the XSD Schema.

During conversion of the schema, the following warning message is shown:

xml2rfc/xml2rfc/data/v3.rnc:495:45: warning: cannot represent an optional group of attributes; approximating

Now, I used an online validator to validate the XSD obtained using trang, which confirms that XSD is syntactically correct.

If, however, using the same tool I try to validate an XML file, the following error messages are raised:

- Element '{http://www.w3.org/2001/XMLSchema}attributeGroup', attribute 'ref': The QName value '{http://www.w3.org/XML/1998/namespace}base' does not resolve to a(n) attribute group definition.

- Element '{http://www.w3.org/2001/XMLSchema}attributeGroup', attribute 'ref': The QName value '{http://www.w3.org/XML/1998/namespace}lang' does not resolve to a(n) attribute group definition.

- Element '{http://www.w3.org/2001/XMLSchema}element', attribute 'ref': The QName value '{http://www.w3.org/2000/svg}svg' does not resolve to a(n) element declaration.

Same thing happens when I try to parse the XSD schema using the lxml python library.

Does anyone have any idea how these issues can be addressed?