You most likely keep it.  First of all, a genuine battery for this model will cost around 165$ + Taxes, a third-party battery can be purchased in one-third of the price. Another fact is that you may often run into troubles like battery not charging, Swell or battery dying anonymously even after having full charge. All in all it's a gamble. I call it Lithium Lottery and trust me I have bough 100s  of them in the past decade.  So if it's charging fine, giving you a decent backup keep it. No point experimenting further.    Answering your main query : No its not normal for a Genuine Battery (if it is) to loose this much capacity in few days   
I installed a new battery 4 days ago (dell xps 13 9333), I did about 10 full cycles since (just to break it in and test it), battery health was 100%, today it dropped to 96%, is it normal? I'm worried... 
New Battery Degraded Fast