Ok Google from outside the app
"certain devices" = devices that meet the requirements (eg. android version)

Now to your problem. It could have many reasons. I assume it's a bug in the Google App. The Samsung Galaxy S6 had the same problem after a system update because of battery saving settings. Google fixed that. I suspect that some battery saving related settings changed with the system update and now the google voice is no longer available outside the google app. I recommend you to contact google support to solve this issue.