@Badwolfe Mangadex isn't behind. Rather, the entire manga is behind the source material of the series: the light novel. That's where the spoilers come from (to put in perspective, the light novel is around 4 times ahead of the manga in terms of content in official english releases alone, japanese ones are even further).
If you want to read it, the manga is currently at volume 5 of the novel, but it's not recommended to start there since the manga doesn't cover the other characters of the story, and 5 is probably the worst volume to start at since it's a big climax that had been building up since the beginning. Ideally you should start from Volume 1, but if you really don't want to you can arguably start at Volume 2 instead, you'll mainly only miss some details about the other characters' early life and some world-building, as well as the introduction of one character but he only becomes very relevant later anyway, at which point he'll be re-introduced again. Or just wait for more episodes of the anime, as they're adapting most of the missing content from the manga, then once the anime gets to the spider meeting the vampire baby, start at Volume 5 to get ahead.