There is a limitation on manga that can be accepted to the site, notably they should works that were first published through a reputable Japanese publisher. Doujinshi is an exception, please refer to that section.
Example magazines include things such as:
Works published on sites like Twitter or Pixiv’s non-comic section are usually not accepted unless they are titled works uploaded by an author already in serialization, in which they’ll be classed as doujinshi.
Any accepted works from Comico or similar webtoon portals should be from the official section only, and not the self-publishing challenge section.
If series originally self-published online through platforms like Twitter or Pixiv (considered doujinshi) are later picked up by a professional publisher and released without significant change, any existing entry in AniList under the doujinshi format would be updated to manga by checking the Self Published option. (retaining the original self-published start date).
Senpai ga Uzai Kouhai no Hanashi was first released on Twitter (here) for a short period of time before the publisher Ichijinsha began collating the original Twitter posts into tankoubon (here). Ichijinsha's publication is of the original work itself (with redrawn art for earlier chapters) and not a new story. If the entry did already exist on AniList as doujinshi, it would have been be updated to manga and the cover later updated to the tankoubon volume.
Sanpakugan-chan wa Tsutaetai. was first released on Twitter (here) before the publisher GANGANpixiv began serialization. Like the first example, this also could have been eligible to be entered as doujinshi at first, if meeting the other requirements.
Also like the first example, the serialization is a re-publication of the first chapters with minor adjustment.