I noticed that the flag for chapterNumbersResetOnNewVolume is set to false:
https://api.mangadex.org/manga/991e9f15-8368-4843-a78a-5605918f9c3e
On checking the manga page, and API feed it shows, that both Volumes 1 and 2 have chapters 1-4 rather than volume 1 having 1-4 and volume 2 having...