My simultaneous voice and data just stopped working a few days ago (first noticed Dec 13, 2017) on my LG G4 which has been active on this account for a couple of years. If I have a data app running and make a call I get a popup saying data is not supported during a voice call. And this is with the 4G LTE icon active.
I've made no recent changes to settings and I have verified my Advanced Calling - HD Voice and Video calling setting is still active. The last system update on the phone was June 7, 2017 with an update to VS9862BA. I've rebooted the phone and toggled data off/on several times and still experience the issue across multiple cities during my daily commute.