It's because you're turned your datetime type variable into a datenum and plotted that. Datenums are the number of days since January 0, 0000. animatedline is not datetime aware insofar as I'm aware. You're probably going to have to format your own XLabels to use date strings for find a helper function to do the relabeling for you.
Or, use a standard plot instead of animatedline since plot is datetime aware.
A related issue: https://www.mathworks.com/matlabcentral/answers/541319-change-uiaxes-datenum-to-datetime-in-appdesigner-for-animatedline-plot