Galaxy S10 and I'm having the same issue. As previous posters stated, I'm posting in the hope that Samsung sees this and can come out with some sort of update to correct the issue. It's a small but annoying issue.
I really haven't tried anything because I truly believe it's a software issue and something an update can fix.
I will say that I thought maybe it was my bluetooth causing the issue because it's a second hand or third party earpiece but it's not the cause, I was able to rule it out.
I have the same problem. I think I found the fix: there seems to be a bug in the "silent" notifications. When you tap-hold an individual notification in the notifications slide-down, you can select Alert or Silent. Every time I noticed my notifications volume was set to off, I found a "silent notification" in the notifications section. I have been turning them off for apps individually as the occur, and it seems to be resolving.
tl;dr: There is a bug for silent notifications, turn off for each app that is set. Look for them at the bottom of the notification panel under "silent notifications".
I also had this problem and after all the solutions provided, it still was defaulting to silent or vibrate.
My solution lay with Bixby who had set up a schedule for me that if I went to work, it went to silent!