I have seen these phenomena in Mangago that series tend to get poor ratings when there is no chapter uploaded. Since chapters take time to upload, it resulted in the first chapter getting poor ratings as a result. It should start to stabilize the more chapters get uploaded, and it would reflect the quality better. Though I feel in general, manga tend to get lower ratings than webtoons, so I take that into account when I read them. An 8.5 rating for manga first chapter is pretty decent.
I do that too, but after being in Mangago for a long time, here is some general impression I have for the ratings in this site.
- Longer series tend to get higher rated than shorter ones. So if you see stuff with just a few chapters but lower rated, it might not be bad. However, stuff that have a lot of chapters but are still lower rated tend to be bad.
- Korean webtoon in general tend to be rated more favorably than manga with similar quality. I'm not sure if this is because webtoon is colored or if it's because they are typically longer. Maybe it's both. This is why I tend to give manga more chance even if it's lower rated. There are some hidden gems.
- Some subject matters get rated more favorably, while others don't. This may not necessarily reflects the quality of the series, but more so what is morally more accepted. For this type of stories, I tend to check the comment section to gauge whether it's stuff I could tolerate. If the comment section in general is positive, I tend to give it a try.

what's with the low rating when we only have 1 chapter and it's actually good?