jven27 microG on GrapheneOS is not supported. The supported method for GMS notifications is an installation of Play Services from GrapheneOS App Store app. By using microG you aren't avoiding using google code as it actually uses google libraries loosely glued together.
A proper solution would be not to fix microG, but to remove it and install Play Services from GrapheneOS App Store app. Play Services on GrapheneOS is sandboxed (not privileged), meaning it has access only to what other apps have and nothing more. It's not invasive. You don't even need to login into Play Services for the notifications to work.
jven27 Yesterday I did have an issue with MicroG and had to reinstall it. Could this have something to do with it?
Yes. Some apps check for access to 'notification service via google' only on first launch and if you have Play Services installed, the apps will 'subscribe'. If you reinstall Play Services, the subscription won't work anymore. You need to reinstall your apps (or clean storage, which is basically the same) and your notifications will work again.
BUT since you're already in this situation with reinstalling, consider do it right. Remove microG, install Play Services, and reinstall apps. This configuration will be supported by GrapheneOS.