Sometimes getting 200 http status code (I think) when going to the next chapter if it not available yet

[Speedphoenix]

When I go on https://mangadex.org/chapter/565107/16 (which is the las page) and go to the next page, instead of saying the chapter isn't out due to the group's policy, it shows a single box that says "200" (a bit like when we get 502)
the only element inside the

reader-images

class is

<div class="alert alert-dark text-center m-auto">200</div>

this does not seem to be consistent with all series
Sometimes happens with the second unreleased chapter if there are multiple pending ones (try one piece for example)

happens on both Chromium and firefox (linux Ubuntu 18.04)

edit: this seems to be a problem specific to some chapters (mostly uploaded by Jaimini it seems?)
for example try reading chapter 15 of Bokutachi wa Benkyou ga Dekinai