I was opening a Solstice box, when I saw 3 icons, Summer Confetti, Winter Confetti, and a Weapon Slot icon. After opening, I looked in my inventory and there was no weapon slot there, and the ones I currently have activated did not get extended. I only received the confetti. I don't know if it was lag, or maybe a bug with them not giving slots if you have activated slots maxed out already.