This is a serious design flaw, that Mossberg should investigate and address right away, before it can bite them too hard.
Because of the design of the plastic stock insert, it 'spreads" side-to-side as the action screws are torqued. There needs to be more support to keep it from spreading. - It's got to be beefed up a little at the rear.
To make room for the plastic insert, the stock is very thin at the point that is put under all of that spreading pressure. - This adds up to cracked stocks.