It's not Samsung. I set a reminder in my Pixel 2, GA said it set the reminder, and nothing happened 5 minutes ago, when it was supposed to go off. It's Android. (And since you're not on the Google January update, it probably goes further back than that.)
Send feedback about it. (Settings -> search for feedback.)
Change that. I normally run with my screen off. When I turned the screen back on for something else, I got the notification (late, but I got it). Evidently, the screen can't be off (or locked - since I use Quick Reboot to blank the screen, I'm not sure which it does).
(How those two answers got cross-connected is beyond me.)