I see your reasoning but March is not exactly early in the year, Xbox One X sales dropped off a lot by March 2018 iirc, and it was behind PS4 as it is behind similarly March 2019. Maybe someone can dig up the figures for comparison.
Yes, maybe someone has the figures at hand. My best recollection is that Xbox was up +30% to 40% y-o-y for the majority of 2018, well past March. I don't think the bump tapered off in Feb. I remember being surprised at how sustained it was. I think it went into the summer months, maybe beyond. I think the positive press about the X, and maybe Gamepass as well, helped sustain the momentum. But I could be wrong; I don't have the figures.
You would expect less than a big YTD drop like that, you'd actually expect PS4/Pro to be dropping faster as there hasn't been a new PS since the Pro and the XBOneX is obviously far newer. That, and we know Sony doesnt seem bothered to do many promotions for their hardware whilst Xbox has been on constant discount/deal. So whatever angle you look at it, Xbox sales are in a concerning decline in their strongest market.
Well, Sony has something going for it that MS doesn't: great exclusives. It's not all about hardware (as I'm sure you know).
p.s. I don't mean to imply that the y-o-y drop for Xbox is unrelated to the Xbox's problem of lacking good exclusive content. I'm sure that's part of the reason. I'm just trying to put the drop into context; I think some of it is because the point of reference is 2018, which was an "up" year for Xbox.