Were you able to resolve this?
I'm working on a deployment that uses the splash page for guests on 17.10.2 and even on a fresh network where only the access control has been switched on the clients are directed to a splash page but it's completely blank. Even inspecting the HTML source code it's also blank. Experienced this on Windows and Android Chrome, Edge and the built in browser on Android.
I've also tried the latest Beta Firmware 18.104 but no change. It looks like it is something server side so I'm surprised reverting back to MX15 firmware worked for you.