Samsung has confirmed that my new GW5 Pro has a known bug which will hopefully be resolved in the next update (I'm currently on UI:4.5 / SV: R920XXU1AWB1). I do not know if this affects everyone or if it's random, but if you are experiencing the issue, when using "Raise Wrist to Wake" the screen will only display for 4 seconds and then goes back to sleep, regardless of your display timeout settings.
Samsung's Temporary Solution
The only way around it at the moment is to interact with the watch and then it will stay on for the set timeout period. Or turn off "Raise Wrist to Wake" and press a button to wake, and it will then stay on for the 15/30/60 seconds you have set it to.