There was a Java 8 backport of ALPN support in Java 8 251/252. Also see the release notes. The reasoning was explain in an email.

Since these new methods exist on the newest versions of Java 8 there is an attempt to use the JDK9AlpnProvider. If the SSLEngine provided however does not implement these methods there would be issues. A property called io.undertow.protocols.alpn.jdk8 will be introduced to disable this check for SSL engines that run on JDK8 and support the new methods.