Hey, I've got the same issue as chinthakaru. I've tried to solve this for quite some time now and still no success. Here's what I tried so far:
-Turned off any energy saving feature.
-Turned off optimization for Outlook.
-Uninstalled and reinstalled Outlook after turning those off.
Here's the thing now:
The notifications work for a short period of time after reinstalling! Every time! To further test this I made an appointment each day at the same time to notice when it stops sending notifications. So today at Thursday it stopped, I reinstalled Outlook last Friday.
That means if you want to use Outlook on your S10 (because that's what I have) you have to reinstall this app every week. And because you can't save your accounts you have to add all accounts manually after reinstalling again. This is absolutely unacceptable and I'm getting the feeling this is not an Outlook but an Android / Samsung issue because I had the same issue with notifications on my S6 edge I had before and I tried many different calendar apps back then.
Edit: And before you even start to ask if I turned on notifications and turned off optimization or anything similar... I tried everything possible in the settings. I'm getting really disappointed because I don't understand how a basic feature like notifications stops working after some time!