Could we add some tests covering both the usage with a singlefield-attr element and with multiple ones, to ensure that both cases works, as it does not use the usualfixXmlConfig() call converting the XML singular name to the plural name used in formats passing an array directly ? I'm still not convinced it works fine.
those tests should be added in FrameworkExtensionTestCase with the fixtures in the 3 formats ensuring consistent compat

Test case added (I did some cleanup also)