so the author has no issue doing this mini series on the side, yet the main series is only a monthly release? :/
This and the main series are released in the same monthly Manga Magazine "Gekkan Dragon Age". Even if the author make more than one chapter in a month, the manga will be updated one chapter per month (with the issue) and as Microwave said, Different artist.
This one sometimes got 2 chapter in one issue though, like these two, chapter 7 and 8 are both in last November's issue. That's why in chapter 7 it's read "To be continued in the next page" instead of number like chapter 8 (basically read TBC in month 12 (December)(Which I should've erase)). This is also the reason why I want to upload this together.