Another Mopar Off My Bucket List - Barracuda Fastback

Sounds like a chip of the old glass may have got over looked and left in the rubber channel.
sorry to hear this. Sharp edges are normal, jagged are not. It may have been bumped on the edge before install in that area. It would give it a place to start a crack. Ugh.
By the way I really like your helmet. I hope to do one up 70s someday myself.