There are degrees of expectation, as you know.
This pretty much nails it. It's just badly done, so it breaks the illusion of form for more people than usual. People are going to have different thresholds for how easily the illusion is broken for them; I've spent enough time drawing drapery and clothing that pretty much every use of a flat pattern on clothing breaks the illusion for me and I just roll with it because I know how much time and tedium is involved in hand drawing patterns to fit the folds is, but the fact that people who aren't aware of the practice are noticing is an indication that this didn't quite hit that level of 'good enough' not to be visually distracting for the average reader. Maybe they were rushed for time, who knows. A square grid is a more difficult pattern to pull off too.
Ironically the dress doesn't really feel like it needs a pattern, it's got enough shapes to it to be visually interesting on its own.
Only chapter 3 and I’m emotionally invested.
Me too. They got a nice personality dynamic.