Eclipse shows XML errors even with all validators disabled and an XML Catalog defined

Question

I have just upgraded to Eclipse 2023-09 from 2020-06 and I'm getting a lot of XML errors. For example I have a custom XML with an XSD. I have added that XSD to the XML Catalog of Eclipse, in the Preferences I have also checked "Suspend all validators" and performed the "full rebuild", but I'm still getting errors.

The error at line #1 is:

vc-elt.1.a: Cannot find the declaration of element 'mapping'.

The error at line #4 is:

Downloading external resources is disabled.

This is how I have added the XSD to the Catalog:

And this is a fragment of the XSD:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns="http://myapp.mycompany.com/schema/mapping"
           targetNamespace="http://myapp.mycompany.com/schema/mapping">

    <xs:element name="mapping">
        <xs:complexType>
            <xs:sequence>

I have also several SOAP XML files without a schema that generate errors like:

cvc-elt.4.2: Cannot resolve 'order' to a type definition for element 'orders'.

Any web page about this topic is very old and I have even found this bug about validators that has still "Status: NEW" after many years.

What's happening? My projects work fine, so I dont' want to see these XML errors because they could hide real errors. Please, help!