[Mission cancels when someone joins]

I'll start a mission and the second someone joins during the 10 second countdown, the mission is cancelled. This is followed by about 10 seconds of awkwardness and confusion and then people just leave the squad. I then go to reload the mission and instead of it doing a 10 second countdown and matchmaking it will start the countdown paused at 5 seconds. I will the resume the countdown and load in. No one can join the mission until after I load in. Is there a cause for this? It happens atleast once every time I hop on to play. This has been going on for as long as I can remember but just figured it was a well known and would get fixed. But maybe its something I'm doing. Just figured I would check on here. 

For reference I'm on PS4 and my internet speed is 150mbps (relatively great for my area so I don't think that could be it).

Any help would be great.