I had the same problem with my 2016.5 CX-5 GX (November 2016 build) and just got back from the dealer who today installed the G33M-62-310B under warranty. The problem surfaced intermittently about a couple of months ago, and then seemed to go away, then returned. North of Toronto, so cold - possibly a contributing factor. It really pissed me off when we had a mid-January Town e-waste event day and I had to load whole bunch of old TVs, CRT computer monitors, computer towers etc. into the front and back seats as the thing chose not to open that morning.
I was lucky that it wouldn't unlock in front of the service advisor last weekend when I brought it in. Initially it worked fine for her, though not for me at home before I headed to the dealership that morning. When she suggested just lubing it and that she "needed to see it" fail firsthand, I thought I was SOL. I tried it again, and luckily it would not actuate and I had her try too. For the assessment, the tech had to open it from the inside after lowering the rear seat.
I just hope that this part was the problem, having read this thread after searching on the installed part number on the zero dollar warranty invoice. Could be that I am not out of the woods yet. Other parts/items could be involved it seems. Time will tell.
Part of my reasoning for buying the very tail-end of the first gen ('13,'14,'15,'16,'16.5) was hoping that all the bugs had been worked out. Guess not.
Edit: The next morning (today) it would not open, maybe frozen from the dealer's free car wash and then sitting in my driveway overnight. (huh) It opened yesterday evening when I picked it up after work. Will check it thoroughly tonight as I didn't have enough time before work.