I also had this when trying to create my last FBA shipment – it wouldn’t let me ship the 2 products in question. Both the products are solid plastic (PVC, PU) so it made no sense at all. I just raised a call with SS and they removed the hazmat flag – took about 4 days.
Yes its pretty random. We get it every summer for different stuff, was betting this year it was paper. Instead they targetted different.
So far have heard of umbrellas, purses, underwear, a waterball – filled with H2o – resin ornaments, glass plaques, photo frames, a moneybox, a book and beauty products.
Amazon are a laughing stock regarding their choice of items. The other fulfilment companies do not have the same problem.