You're not imagining it. Facebook deliberately doesn't send you notifications for every new post.
⚡ Get notified in 1-2 minutes of every new post
It is indeed by design. Facebook will not send you a Push Notification for every new post in the Facebook groups you're in.
Instead, it will send you a notification (hours later) of the most relevant posts of the day from the groups you follow.
There's no way to fix this (through the app or phone), because it's not an "issue" - that's just how it is made by design. Trust me, I tried everything.
The only solution that worked for me was creating a chrome plugin that reads my Facebook feed 24/7, and sends me an instant email the moment there's a new post. Of course most users won't be able to do something like that (you need a laptop/pc running 24/7).
It's ridiculous that Meta didn't already have this built in. But now you can at least rest easy knowing it's not possible to do this without using some third-party tools.
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Get Instant Facebook Group Alerts⚡ 1-2 minute alerts • ♾️ Unlimited groups • 🤖 Auto-commenting