Help with Burnup / Burndown ?

I'm trying to build a burnup or burndown by feature. I know that's not a native feature in ADO right now, so I built it using tags.

Even though most of the stories were written (and some completed) well in advance of May 17, I created the tag on the 17th, and it appears they're being plotted against the last changed date.

Am I doing something wrong, or is this known behavior?