That is super interesting! Definitely seems like a functionality dependent on the device. When you open up the even on the other phone, does it say that there is an alert set up? If so, can you modify it at all?
This probably won't be any hep but I had an issue with my old Galaxy Avant not syncing up to Google Calendar. I attributed to the fact that the Avant wasn't supported with updates since Android 4. The problem went away when I got my new Moto G7 with Android 9.
Good morning, shipshep!
I just wanted to swing by to see if you're still having issues with the calendar being shared between both of your Galaxy S10 devices. If so, please take a look at Chris' reply above and get back to us.