I noticed this occasionally on our January Explorer OTS cruise. It worked most just after sailaway from each port. Then it was definitely turned off. I could even iMessage friends not on the ship during those times without buying Voom. (And it wasn't data, because it was on my iPad, which doesn't have data.) Not frequent enough to rely on, for sure.
And the $2/day Royal App chat feature isn't set up correctly in the Apple App Store as a "direct messaging" app, so you won't get notifications unless the app is open. So it was useless.