POSSIBLE SOLUTION i was having the same issue. i was getting little numbers in a red bubble next to the app but no notifications were coming through. i would have to actually click on the app to see my messages. and sometimes i had missed important messages from friends and family. it had been like it for 3 days, my phone worked fine before. at first i thought maybe it was the app having issues, however my brother could use facebook messenger fine on his samsung phone. then i thought maybe its my account, so i decided to log my messenger account into his phone and see if notifications would come through and they did. this made me think it was an issue with my phone. so i tried resetting everything, changing power saving options, every possible solution posted on the internet i tried. still no joy. literally i felt like the problem would never be fixed. then my brother singed into his messenger account on my phone and he was getting notifications which puzzled me because then it couldnt of been the phone right? then i signed into messenger on my tab A and i was getting message notifications. so i had a hard think about what it could be, then it hit me. basically all these times i had signed in other devices and my brother was singed in my phone, they were only singed into messenger and not the facebook app as well. on my phone im normally signed into messenger and the facebook application at the same time. so then i disabled just the normal facebook app. and restarted my phone, signed into just messenger and i started getting message notifications. i assumed maybe it had fixed the issue so i redownloaded the facebook app and logged back in and had both running on my phone again however, i wasnt getting messenger notifications again. ive come to the solution that if the facebook app is disabled and i am just logged into messenger i get message notifications. so i assume its a clash between the two apps. i guess the only solution till an update comes out is to just log in on chrome browser to check feed and make a home page short cut for facebook (via chrome) instead. i hardly use feed anyway only to post a picture every few months to prove im not dead. if youre an active facebook user and scared of missing something then you could just change your facebook account settings to receive email notifications then log in the browser to view it.
i also deleted messenger and used messenger lite instead and that worked even signed into the regular facebook application. however messenger lite is too basic as i normally video call family. i guess its a choice between whether you want a feed and basic messenger or you want the better messenger and just use a browser for the feed.
let me know if this was helpful or worked, thanks.
Kate.