I would like to know the answer to this as well as I am having the same exact problem. I had my phone in my pocket for hours today at work and had a message just sitting there with no notifications so I didnt get it til hours later.
Having same issues as OP with same steps taken to eliminate what may/may not be. Issue is only with the messaging app and started with recent late December update.
Post update, I noticed a change in messages that I might have recently replied/viewed saying something along the lines of "bookmarked unread messages here". Even if exit that conversation and close the app (task close & even force close/clear cache) the problem with intermittent / lack of notifications persists. I've even wiped cache in the bootloader menu.
Model is Samsung SM-a102u
Was this ever fixed? I have the same issues as the OP and I've followed all the advice and steps on this feed. It does not work and I'm the only one on our phone plan with this issue so my phone company says it has to be my phone.
@userS9uytJgLrq If notifications work in Safe Mode, there is a third-party app preventing the notifications from being delivered in normal mode. You will need to determine which app is causing the problem. This can be done by uninstalling one app at a time until you notice the issue is resolved. It is recommended to start with the most recently installed apps.
SMS Backup and Restore, which I have used on many phones for many years without issue, and has let me transfer sms to new phones.
Uninstalled and notifcations worked.
Now I remember that when you backed up it asked to be set as the default messaging app. On every oher phone it must have automatically reset to the normal messaging app when finished. Not on the A10.
After re-install I tried a backup and the current version does not ask you to set is as default.
My A10 had the problem for a over a year - through several upgrades of SMS Backup and Restore. It took uninstall-reinstall to clear the problem.