I have a weird one for you!
I have a Macbook and an iPhone XR. None of my reminder alerts happen on my iPhone. They do happen on the Macbook but obviously I don’t carry that around in my trouser pocket!
I wonder if this is a bug or could it be that there is a way to set the ‘Primary’ device?
So it seems that it takes some time for the change to take effect. I’m now receiving notifications for reminders. Thanks to @Wolfie for his suggestion. Worked a treat!