for those still doubting if this ended or not, bookwalker use tags and let you search with them, and this manga have a very specific one which is 完結 and it means that the work is completed/finalized the serialization, so yea it did end, and the 3 volume adapted Arc 1, here example http://puu.sh/EPpIL/b109a5b930.png green tag is 完結 so is completed with 3 volumes (volume 3 out december 26th, but for those that don't know how it works, it include already published chapters, 10, 11, 12 and extra and bonus, the actual end not until the extras so uh chapter 12.5?) here another example of completed known works that use that green tag 完結 https://puu.sh/EPpKO/2ed0bf1e66.png , and here how ongoing works look, since it dont use that tag http://puu.sh/EPpMP/e4503b757d.png , if someone still need proof that this ended with 3 volumes adapting arc 1, dunno what else you will need lol, the completion tag in bookwalker get added once the last volume is available for reservation.
and like i posted in a previous comment, there is this tweet by the artist when chapter 12 was published https://twitter.com/yuzuanko_/status/1154393717177929729 try not to use google translator and read the comments too, there is some specific ones asking if this really ended, and artist saying yes
whether this continue or not someday who knows, but up to date there is no signs of that
