FCM does support notifications when app is closed. Your code seems to be OK, so I suppose battery economizers (or power savers) can kill your notifications. I had such problems on my Asus Zenfone, also they were reported in cases of using Huawei and Xiaomi. Just disable them or add your app in exception list, if there is one. Also there is a new power-saving mode in recent releases of Android, try to disable it too.
