It's commonly reported, always hulu, never launches any other app, the button is never touched, I have 3 ultras and each has exhibited the problem. Intermittent enough (for me) that I haven't trashed them, but it's a problem that Roku doesn't acknowledge.
Please work with just one of your customers with a reasonably reproduceable recurring problem and solve this once and for all.
(I used to subscribe to Hulu and at that time, it launched the app. I uninstalled the channel so it now just offers to install and launch Hulu with the option to exit (abort). I doubt that uninstalling the app is what Hulu intended as the result of adding that button to the remote.)