When looking at ones total collections within Arena it says that the original Ixalan was released in September 2018 when it was released in 2017. Due to(presumably) the way the code works in sorting the sets for the collections page within ones profile, this means that it looks like Rivals of Ixalan is the very first set released almost a year before Ixalan proper. In fact as you can see in the screenshot, it actually looks like both Core Set 2019 and Dominaria came out before Ixalan proper as well which makes the whole thing look a little silly. Now is this a major bug? No. Is it probably really easy to fix and ridiculously reproducable? Probably. Ill drop my log just in case but presumably this is just a small typo on a page no one looks at that no ones bothered to say, hey wait a second that doesnt make any sense.