@sventle it's not always the case, but some oneshots are put out as kind of a test, if enough interest is shown it can become a series, and I've seen several where they do. and even if it's not specifically released for that purpose, it might be out of habit if an author has done some of those or is used to writing ongoing series as opposed to oneshots. or it could be the author meant it to be more than a oneshot but had other obligations, maybe another series came up and got in the way.
but yeah, I agree, that ending does not feel like a oneshot ending, one more page, hell one more panel even, and it could have been ended a couple different ways, but it just cut off instead.