YouTube is now promoting YouTube Red original content with full-screen ads within the YouTube app, in what is probably an attempt to get more people to view the content that's put out. There doesn't seem to be a rhyme or reason to when the ads pop up but it's reported that they should be showing up first thing when you open up the app, so it's highly likely that not all users will end up seeing the full-screen ads the very next time they open up YouTube on their phones or tablets. It's also possible that Google could simply be testing this new full-screen ad scenario and that they may not decide to keep doing it in which case some users may never experience them.
While it wasn't explicitly stated that the users who are receiving these ads are subscribers of YouTube Red at the $9.99 monthly cost, it seems to be implied that that's the case, and if so then that would mean that Google is using ads to promote the original content in the version of the app where users are paying a monthly fee to avoid such things. Google could also end up using this type of full-screen ad for non-YouTube Red subscribers as a way to make it more visible that this sort of content is available, but judging by the picture this certainly appears to be a user who already subscribes as the ad gives an option to "watch now" or hit the no thanks button to get rid of the ad and continue to the YouTube home screen, and presumably a non-subscriber wouldn't have the watch now option available.
When exactly Google started inserting these ads is unclear but it looks like it might have been recent as this is the first time that any such activity has been reported by a user. It's also unclear how often Google might decide to insert these full-screen ads if they do plan on keeping them around for a while, though it is worth noting that users who pay the $9.99 a month for a zero-ad experience may not particularly care to see these show up, and if enough users complain Google could simply stop using them.