Seems it was a typo on the Beta Software download site. Apple since then has corrected the version number of the watchOS 6 GM Seed to Build 17R575. So you should be up to date with your Series 4 watch.
Software Update says "Apple Watch is up to date for me". I have the latest profile installed on the watch, and have tried the update from the watch's settings and the iPhone Watch app's settings, and both get that result. It is currently running 17R5575a. What's the trick to getting it installed this time?