I think the first set of chapters had the sepia naturally. The last group worked off hi-res B&W and added the sepia after the editing was done, because people liked it so much.
The original translators were using the twitter raws that are naturally sepia and still are, as can be seen with the same chapter here: https://twitter.com/mikanuji/status/1163477838143451136
However, the volume releases are in black and white, (for whatever reason) those are the ones being used since Secret Society Blanket took over and now WindyFall.
This may be my first comment ever on Mangadex, but yea, keep the sepia. I think it's a nice part of this series' unique style (even though I've read plenty of the author's 'other' work not in sepia).
@Lilliwyt Yeah seriously. For all the talk that Iori keeps getting more abusive, this is such a sweet and simple chapter. You'd think anyone else might have appreciated it enough to say so.