Is it the same in your case?
The notifications are allowed
It's allowed to use background data even when data saver is on
Battery usage is unrestricted
I don't or rarely clear cache. Do I need to clear often? Is that a norm these days?
Other settings, I didn't change. Do I have to make it appear on top of other apps? Is yours set to appear on top of other apps?
Do you always receive Facebook messenger notifications on time? This has been an ongoing issue since the day I got this phone.
No problem with my FB messenger, all notifications received on time.
I notice your messenger is outdated, my version is 379.1.0.23.114. Try to update to latest version and yes, clear cache for your most used apps every month or whenever you like. I clear cache for my most used apps if I have nothing to do 😂
I notice your messenger is outdated, my version is 379.1.0.23.114. Try to update to latest version and yes, clear cache for your most used apps every month or whenever you like. I clear cache for my most used apps if I have nothing to do 😂
are you using a Fold? I think you have a different UI. There's no "apps and notifications". I think I saw it like a decade ago? in case you've missed, the notifications are allowed for the app. Can you see a photo or just text? sorry I'm not sure if you're an AI or human. 
It's working fine on my end. Aside from the settings I've mentioned in my original post, the only things I've added/changed were update messenger via Google play store, and set it to appear on top of other apps.
You might need to check "meta" apps in the settings if they are enabled. My "meta services" is set to have unrestricted battery and background data usage. I don't remember setting that up like that but maybe you need to try to check and set it up like that if changing the settings of messenger doesn't work.
Compared to the time I wrote this post, I'm now always on power saving and data saving mode, but my messenger app and location services still work normally.
I still keep the Bixby routine to prompt opening messenger app every night though I must admit I sometimes just dismiss it because I don't have this issue for now.
I stay logged off from all my Facebook accounts and Instagram but I still update those apps when prompted by Google play store.
Are you not receiving any message sent to you by anyone? I have a lot of people blocked from one of my messenger accounts. My DMs from there are just fine but group chats were late. Some members of my chat groups are the people that I have blocked previously. Well now that I came to think of it, perhaps having an ex or someone you have blocked in your group would delay notifications? idk ahaha!
I have created another Facebook account and messenger dedicated to group chats and I don't have any of the members blocked there. Perhaps that's the reason why I don't get delayed group messages. I already have disabled all group chat notifications from my other accounts to avoid redundancy.
If I were in your case, I'll update and change my Facebook messenger app settings first then observe. Then, if needed, check the meta apps.
You might need to check "meta" apps in the settings if they are enabled. My "meta services" is set to have unrestricted battery and background data usage. I don't remember setting that up like that but maybe you need to try to check and set it up like that if changing the settings of messenger doesn't work.
Compared to the time I wrote this post, I'm now always on power saving and data saving mode, but my messenger app and location services still work normally.
I still keep the Bixby routine to prompt opening messenger app every night though I must admit I sometimes just dismiss it because I don't have this issue for now.
I stay logged off from all my Facebook accounts and Instagram but I still update those apps when prompted by Google play store.
Are you not receiving any message sent to you by anyone? I have a lot of people blocked from one of my messenger accounts. My DMs from there are just fine but group chats were late. Some members of my chat groups are the people that I have blocked previously. Well now that I came to think of it, perhaps having an ex or someone you have blocked in your group would delay notifications? idk ahaha!
I have created another Facebook account and messenger dedicated to group chats and I don't have any of the members blocked there. Perhaps that's the reason why I don't get delayed group messages. I already have disabled all group chat notifications from my other accounts to avoid redundancy.
If I were in your case, I'll update and change my Facebook messenger app settings first then observe. Then, if needed, check the meta apps.
