Take this "jboss-app.xml" file that works with WildFly 31:

<?xml version="1.0" encoding="UTF-8"?>
<jboss-app xmlns="urn:jboss:jakartaee:1.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="urn:jboss:jakartaee:1.0 https://www.jboss.org/schema/jbossas/jboss-app_9_0.xsd"
    version="9">
  <security-domain>security-domain</security-domain>
</jboss-app> 

Eclipse reports validation errors :

cvc-complex-type.2.4.a: Invalid content was found starting with element '{"urn:jboss:jakartaee:1.0":security-domain}'. One of '{"https://jakarta.ee/xml/ns/jakartaee":application-name, "https://jakarta.ee/xml/ns/jakartaee":description, "https://jakarta.ee/xml/ns/jakartaee":display-name, "https://jakarta.ee/xml/ns/jakartaee":icon, "https://jakarta.ee/xml/ns/jakartaee":initialize-in-order, "https://jakarta.ee/xml/ns/jakartaee":module}' is expected.
cvc-complex-type.3.1: Value '9' of attribute 'version' of element 'jboss-app' is not valid with respect to the corresponding attribute use. Attribute 'version' has a fixed value of '10'.

This seems to be caused by the fact that "jboss-app_9_0.xsd" defines the type "jboss-appType" as an extension of "jakartaee:applicationType". Somehow, this does not validate, but I don't know why .

Other files like "jboss-web_15_0.xsd", where a "jboss-web.xml" validates in Eclipse, don't extend the JakartaEE root type, but declare all elements themself.

Attached is a modified "jboss-app.xsd" which does not show the validation errors:

jboss-app_9_0.xsd

Same workaround seems to be necessary for "moduleType" (which probably should duplicate the content of the base type).

If you confirm this change, I could do a pull request. But I would require a lot of guidance.