There is a way!

I had this same problem so I started testing. For starters, I'm using an S8+ with BatStock 1.20 and BatSock Kerknel 1.2.2
The trick was to format and reinstall, but this time, on the Aroma debloating options, I unticked Bixby voice. Right now, in Settings->Apps->Show System Apps, I only have Bixby Vision installed. No other Bixby stuff is shown there. Most likely, if you remove all Bixby related apps, the button will lose its ability to wake your device!
Now, no matter what I do, Bixby button won't wake the screen, and Button Mapper works great. Only having one issue: Bixby button customization disables on Button Mapper when I restart my device, meaning I have to reactive it after boot. I do have to say I had this same problem when I had Bixby Voice installed and Bixby button able to wake the screen.