So I have 3 sony headphones mdr-xb450ap, mdr-xb50ap and sth40d and I'm using them with xperia xz premium smartphone, about 2 months ago I noticed that smartkey functions don't work anymore on any of those headphones, it only works when music app is running on the screen, but when its on the background, none of the headphones' buttons respond, sth40d also has volume buttons, but they seem to function normally, I recently updated to android 9 but it still didn't fix the problem.
