It's most likely that modern techniques are too "perfect"; the horribly impure coloration and janky glass composition (the texture of the glass chunk in the lit

It's most likely that modern techniques are too "perfect"; the horribly impure coloration and janky glass composition (the texture of the glass chunk in the literal sense) as a result of technological limitations; in contrast to the "new" glass of the 19th century, which was already experiencing the first industrial revolution and thus could produce quality glass a lot more consistently. At least that's the impression I got from googling "medieval stained glass vs modern"