Smart Unlock by location working perfectly for me. Device stays unlocked for 4 hrs as intended.
Smart Unlock for Trusted device also working perfectly. Phone stays unlocked when it pairs with my vehicle, or pairs with BT speaker, etc.
Have plenty of complaints with Samsung's skinned version of 8.0 Oreo but this isn't one of them.
This is not only a Samsung problem. My last phone, an LG, had the same issue. It used to work okay but lately it does not, it's probably a Google issue.
I have 3 devices running various versions of stock android. A 4th (this one) running with a skin. Google issue? I dont think so. It can't be mere luck I have 4 working devices.
Do you have "High Accuracy" enabled for location? This makes a difference.