Achievement unlocking is delayed

When I unlock Achievments, on Singleplayer and Multiplayer, the achievment unlocking notification appears approx 30 - 60secs after completing the Achievment.

As you can see in the video below, the achievment is 'apparently' unlocked when I am in the achievment menu, when I am not doing anything. I couldn't include me doing the achievment in the video as my device only lets me record videos that last for 30 secs and below.

Steps to Reproduce:

1. Try to unlock achievment that isn't already unlocked

2. Notice that achievment notification shows up 30 - 60secs later.

Expected Result:

Notification shows as soon as you have done the required thing to unlock the achievment.

This can make me think that I have already unlocked certain achievments when I haven't