They claim that plastic is not allowed since it easier to refill bottles with liquor but I've never been denied boarding with them.
On the last cruise in Fl. last March, my DH returned to the ship during a B2B with a suitcase filled only with 8 one liter bottles inside, so you can't say they didn't see them.
They never said a word when it was scanned. Nor did they make him go to the liquor check in desk.