Current setup that works:
Wildfly 10.0.0.Final
Keycloak 3.4.3.Final

• Deployed a REST endpoint (/zupplyio-orderservice/orders) to Wildfly 10.0.0.Final
• Access the endpoint through a React app with a token received from Keycloak.
• Returns a 200 http response with payload
• undertow server.log:
  19:35:33,356 INFO [io.undertow.accesslog] (default task-1) 127.0.0.1 [30/Mar/2018:19:35:33 +0200] "OPTIONS /zupplyio-orderservice/orders HTTP/1.1" 200 "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.75 Mobile Safari/537.36"
  19:35:33,994 INFO [stdout] (default task-2) *** Calling getOrders ***
  19:35:34,121 INFO [io.undertow.accesslog] (default task-2) 127.0.0.1 [30/Mar/2018:19:35:34 +0200] "GET /zupplyio-orderservice/orders HTTP/1.1" 200 "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.75 Mobile Safari/537.36"

Setup that doesn't work:
Wildfly 12.0.0.Final
Keycloak 3.4.3.Final

• Deployed a REST endpoint (/zupplyio-orderservice/orders) to Wildfly 10.0.0.Final
• Access the endpoint through a React app with a token received from Keycloak.
• Returns a 200 http response with NO payload
• undertow server.log:
  19:25:22,707 INFO [io.undertow.accesslog] (default task-1) 127.0.0.1 [30/Mar/2018:19:25:22 +0200] "OPTIONS /zupplyio-orderservice/orders HTTP/1.1" 200 "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.75 Mobile Safari/537.36"
  19:25:23,491 INFO [io.undertow.accesslog] (default task-1) 127.0.0.1 [30/Mar/2018:19:25:23 +0200] "GET /zupplyio-orderservice/orders HTTP/1.1" 200 "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.75 Mobile Safari/537.36"

As you can see from the server.log, my endpoint is probably not called in Wildfly 12.0.0.Final since the log output isn't written to server.log