On a hunch I wont get chasticed again with a simple solution people do overlook, do you have any power saving options selected? Always on will not work if there are.
Haha. The issue occured after the sw update. So I did a Google search which included the words... Always on display not working after sw update s8. I wanted to try the vzw post first but here we are. Here's the solution. Apparently the update offers new options. Which need to be set. Here's the link to the solution that worked for me. I'd bet others will have the same issue. https://us.community.samsung.com/t5/Galaxy-S8/Always-on-display-stopped-working/m-p/535047#M28597
Keeping a charge on my phone is something I pay close attention to, as I know how important it is. I can definitely assist you with power saving ideas. Here is a link that is specific to the s8: http://spr.ly/6606EoyEM
In addition, I have included a video that helps with all Android devices: http://spr.ly/6607EoyE3
I also want to recommend a power bank, so you can make sure you have a charge, even when you're on the go. Check out these options: http://spr.ly/6608EoyEO*