I don't have this issue on my Note9. You can try checking those threads https://forums.androidcentral.com/samsung-galaxy-s6/537843-notifications-not-working-when-screen-sleeping-2.html https://forums.androidcentral.com/ask-question/907457-push-notifications-not-working-while-screen-off-2.html
Sorry I can't be more helpful as I don't have an S10+ or experienced this issue or have found a definitive answer to this issue
I had this for a while on my Note 8 and still sometimes happena intermittently. No idea what's causing it and/or how to fix it. I went through all settings and several phone resets with no luck. I think the Nov security update fixed it but thunk the Feb security update bugged me out again. It's weird but you're not alone with that issue.
If I find a reliable fix absolutely, but it was an issue for me that started sometime end of summer/early fall then seemed to be fixed with Nov patch and now back since Feb patch. Really odd but ya if I figure this out I'll update as well. I'll bookmark this topic.