I'm not sure if a thread has been made about this a while back, and I apologize if there was.
A thing I noticed was that in certain maps, you can't quick switch mines anymore. For example, Kill Creek. I'm not sure how long ago this was implemented, or if it's user error. I'm pretty sure I'm quick switching correctly, as I have always done it. You guys know the drill, plant mine, switch to alternate weapon, switch back to mine and plant, repeat.