Just unlocked it during the phantom draft; it must be hard-coded to seven instead of whatever the event maximum value really is. I'm sure doing it here is much easier than in an actual paid event. Unfortunately I can't remember if I lost any games before this 7-0 streak, so I unsure if you actually have to hit 7-0 total or if you just need a seven game streak.
So, this was unexpected and unintended, and we thought at first we should fix it. But, as we talked more about that, we decided it felt too fun-police.
You know what, This Is Fine. If you can go 7-0 in Midweek Magic, that's still worth an achievement. Have fun