In WildFly 21.0.0, created a simple MYSQL (non-XA) datasource with no explicit configuration of validation. The below was automatically added:

<validation>
 <valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLValidConnectionChecker"/>
 <background-validation>true</background-validation>
 <exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLExceptionSorter"/>
 </validation>

The above is problematic

• Background validation is not recommended in most cases as it is less reliable than validate-on-match (since it runs periodically rather than at the point/time connections are requested from the pool). A better default would be <validate-on-match>true</validate-on-match> with background-validation disabled. Background validation should typically be used only if explicit testing verifies that validate-on-match noticeably impacts performance for end-users.
• Using background-validation without background-validation-millis > 0 will not enable background validation.