Guys i just wanted to share my experience with you.
I have both Lumia 950XL & 950 also 640XL, 640XL have no issues with that, but had one issue with charging replaced the connector and changed some things on the motherboard...thats what i have been told but i think the connector was the issue...it is special designed it is MICRO USB but not like the other phones ordered it from EBAY.
My bro uses the 950 and i use the 950XL, at first his phone started to act weird like reboot and turn off randomly, now mine have too...he ordered a brand new battery and now he use it for about 2-3 days moderate user. So i think mine will be the same thing...just get new battery...Yesterday had to charge it 3times and i didnt use it a lot....an hour ago it had around 60% battery and i got it of my pocket and it was black screen couldnt turn it on...i get back home put it on the charger and it started to charge...im using only the original fast charger....at first when i bought it i could easily get 2 days because of the large capacity of the battery, now i just told my bro to get me another one....
And one more thing for all you that leaves your phone to recharge over the night...DO NOT DO THAT...even though it is Lithium battery...i think that is the problem...it takes around 1 hour to fully be charged and you leave it to charge for 6-7 hours more than it should....i started to recharge it when i woke up and wait for it to get 100% and dont use it while charging...that is my experience. Hope it will help someone