Undertow can be configured to serve static content per host via the location resource. This location resource is exposed to the mod_cluster balancer.

               "context" => {
                    "/custom_location" => {
                        "requests" => 0,
                        "status" => "enabled"
                    },

However, all EAP standalone profiles have enabled root location by default. This root location is then exposed to the balancer( Application root "/" is registered).

  "context" => {
                    "/" => {
                        "requests" => 0,
                        "status" => "enabled"
                    },

Root application matches with any application call and therefore mod_cluster balancer is unable to correctly route requests.

To make mod_cluster work, one must delete root location from worker nodes.

/subsystem=undertow/server=default-server/host=default-host/location=\/:remove()

or excluding context ROOT

/subsystem=modcluster/mod-cluster-config=configuration:write-attribute(name=excluded-contexts,value="ROOT")

Proposal:
Exclude ROOT context by default