Well, I was close with the localization guess. The forum you linked has Finnish localization, and I did not realize that the moment.js .day() function is locale-dependent IF you give it a string to parse. I was giving it the string “Monday” (which in Finnish is “maanantai”), so the parse failed and moment.js fell back to Sunday.
I’ve changed this in the below PR to use the number 1 which is always Monday no matter the locale.
https://github.com/discourse/discourse/pull/9737
As an additional note, “Next Week” is just "today + 7 days so locale is not a factor there.