This happens to me and some friends too.
Same GC, same FC, queue together, one gets the DF pop, the other doesnt.
The one who did enter FL re-queue, gets another pop after 5-15min, the other poor guy still waiting for his 1st queue which hasn't popped yet.

It changes every day or so who is the lucky guy and who is not.
But it does happen a lot.





One thing I noticed is that you should exit Frontlines as soon as possible.
And re-queue as soon as possible too.

It usually takes 30+min for me to get the 1st queue. (Sometimes 60+min when I'm the unlucky guy of the day)
But when someone wins FL and I leave and re-queue really fast, I'll get another FL pop in 5-10min.
And it happens quite consistently every day.