See in particular r/SchuylkillNotes I think it is.
I've found a few similar things in NYC, although I don't know if they're similar enough to count.
#hypergraphia is definitely a thing, and (layman's guess) coupled with #schizophrenia it might have led to similar behavior in multiple people independently.
@ceoln I’ve been following this for a while. I’ve been on this subreddit as well. https://sharonahill.com/mysterious-notes-hidden-in-food-boxes-continue-to-be-found-in-pennsylvania/