Brilliant, my friend!
FIXED! Thanks everyone for the comments. I had not yet changed/deleted/reinstalled the Watch Roku activity or the Roku device on my Harmony hubs, nor did I need to. The following change on the Roku’s solved the issue: Settings > System > Advanced System Settings > Control by Mobile apps > Network access. Change the setting to “Permissive”. I can once again control the Roku via the Harmony Hub remotes and the Harmony iPhone app.
THANK YOU!!! Glad I found this post. I was having the same problem and tried everything you originally did to try and make it work. I couldn't tell if the problem was my Roku 3810x or the Harmony Hub. Almost was about to give up and resolved to use my Harmony to turn TV and receiver on and then the Roku remote to control everything else. Found your post, made the change to "permissive" and, voila, all good now.
This solution also works for Control4 systems.