Jastow defaults to using an 'ISO-8859-1' encoding. In EAP 7.4's undertow 2.2.19, that new included request URL's query is decoded by QueryParameterUtils and this instead defaults to a UTF-8 encoding internally and may be influenced by the connector url-charset, but this also defaults to UTF-8. The latest upstream Undertow 2.2.x is changed a bit to use a DispatchUtils class to handle the include and calls io.undertow.util.URLUtils for the query string decoding but that is also similarly influenced by the url-charset with a UTF-8 default
So there's a mismatch and disconnect there from the jastow include inherently encoding the url parameters with ISO-8859-1 by default and Undertow trying to decode it with UTF-8 by default. This can cause character corruption of query parameter characters in included requests as ISO-8859-1 is used by Jastow when encoding the included URI but UTF-8 is used by Undertow when decoding it in the included request processing.