According to Coffee Stain, there now is 2 versions of the dedicated server for Satisfactory, early access and experimental. Right now they are the same since update 5 just went to early access, but that will change in the future. If you want to run the experimental branch, you now have to add
-beta experimental
to the steamcmd line. See https://satisfactory.fandom.com/wiki/Dedicated_servers#Experimental_Branch_Selection
It looks like you should be able to just add the -beta experimental to the game id in the UNRAID setup screen so that GAME_ID is "1690800 -beta experimental", but I'm not sure how that'll work. Personally, I'm going to not worry about it until the experimental branch diverges from early access again to mess with it.