Extending the deadline has no detrimental effect. You may add time and modifications to satisfy a customer. However, speedier work benefits both sides. Once they accept delivery and start rating and reviewing, the buyer may see how many days it took to finish the job. I recommend communicating and explaining to your customer while extending the time to keep them happy.
I guess it depends on your gig categories, gigs, and profile performance.
If you're not outperforming your competition, they'll get the briefs, and I suspect they get a few.
Continue to complete orders and perhaps you'll get briefs again.