What has happened that is allowing this late entry into missions so often ? I have had people join when the mission is all but finished , so being nice , halt the extraction so the timer isn't tripped so they can get their relic filled with 10 traces . For my own selfish reason , it gives another chance at the drop table . Doesn't always work tho , noobs will trip the timer cos they think its a race , others just don't give a damn .
The game will allow you to join up until the game has reached a set "point of no return" (entering a spy vault, capturing the target, etc). However, once the game has allowed you to join, a player may take some time to actually load in, depending on their and their host's connection speeds. Therefore, they may actually appear in the mission well after the "point of no return" has been passed.
The issue with traces is that all traces that dropped before a new player joined cannot be picked up by them, so they may not have any opportunity to collect all ten.
It's a system that needs fixing. Once that's done, we can all go on and complain about all the speedrunners who won't wait 45 seconds for a chance at another reward.