Has happened to me on both normal and legend runs of onslaught but when your team goes through the portal, the room to fight the boss won’t open. The 3 pyramid like objects that your team usually shoots to open the boss room bugs out when 1 of 3 will not register being shot, hence the boss room not opening. It’s just annoying when it happens on wave 30 onwards during a legend run because your effort has now gone to waste.