I too experienced this with my Roku 4 and assumed it was broken. We use the headphones VERY frequently when our kid goes down so it is a crucial feature for us. Assuming my Roku 4 was broken I ordered an Ultra, but when it arrived we had the same problem. I called Roku and they said it was a known problem. VERY disappointed with Roku, how they have dealt with this and how slowly they have dealt with this. I am an amateur installer (friends and family) and always recommend Roku, but I have been hesitant as of late due to this issue and the customer service. The woman I talked to didn't know as much about the system as I did, and when I asked her if she owned a Roku she said "no". I have been unplugging my Roku almost every night to "reset" it... this works, but what a pain. Returned the Ultra. Thanks for whoever got the information that they are going to repair it with the next patch... I sure hope so.
My Roku 3 is now fixed after the 8.0.0 update at the end of October.
I can now use HDMI ARC to connect sound bar to tv. And all the controls work great, including headphones. Now I can use the Roku remote for everything except turning off the TV. Or I can use the Samsung one remote for all.
It used to not come off mute after removing headphones on ARC, so I had to run the old school optical and use 3 remotes, lol.