From my profile:
It's the same with months:
Happy new year.
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I don't think this is a bug, actually.
After closer inspection, looks like the actual reputation value for the written date is to the right of the text. Without vertical graph lines, it's hard to notice, but it's pretty clear in the graph of this user:
As can be seen in their reputation tab, they had 275 rep in December 23 (2018), where the vertical red line I added is located. Based on that same logic, the actual rep for the last date is the end of the graph.
So nothing really "goes into 2019", the graph ends in 2019, and since in your case it spans several years, it can't go into actual dates as in the above, where the user is member for less than a month.