When in the pre-game lobby, open your inventory and try to switch weapons/loadouts. The effects will not persist once you have left the inventory.
Occasionally the screen will go black with the spinning loading symbol as well. This prevents you from changing your loadout or readying up. Once the timer ticks down the mission starts as normal, going from black screen to the “in-flight” loading screen. This appears to happen after an attempt at switching loadouts and was also a problem pre-patch 10.