It's strange why you decided so. Are you able to provide links to the original authors for comparison? I just see a different picture here; other authors use his scenes.
It is quite possible that the slightly reworked scenes that they pass off as originals do not belong to them at all.
In this case, if the author is informed about this, he can reconsider access to his prices or stop giving them to the public at all.
Also, regarding other scenes on his Pixiv, yes, some of the animations that he provides there belong to other popular authors such as HC Giddora Imsayaa, but he does not post them to the public, but only provides video material.