Mm, I remember encountering terrible punctuation in fanfiction (including that exact dialogue mistake) as a snobby 12-year-old, so I don't think this is a recent problem. I can't remember my schools teaching punctuation specifically, either (this would be most of the '90s and early 2000s). I know I picked up on most punctuation conventions unusually early just from being a voracious reader, and lucked out genetically on having a brain that remembers language-associated things easily, so I was always a bit mystified that everyone else didn't just absorb what they saw in professionally-edited books and publications.