So my s9 on android 1.0, the stock reminders app worked perfectly but randomly stopped showing notifications, so I chatted Samsung and they said clear app data restart phone all that nothing worked so then I tried clear cache partitions and that work but after a few days stopped again. While I was waiting to have time to chat with samsung I downloaded an alternative reminders app it worked but also stopped working
I would recommend wiping the cache partition of your device.
Here are the steps:
1. Turn off the device.
2. Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
3. When the Android logo displays, release all three keys.
4. An ‘Installing system update’ message will show for 30 – 60 seconds before the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press the Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.