Mine was doing this for a bit! One day I noticed the car felt sluggish so I checked the wheels and found the left rear was a bit warm. It was clearly the handbrake mechanism causing it, but I didn’t have time to do anything about it so I started pushing the lever on the calliper back to the end stop by hand when releasing the handbrake. After a week or so of doing that it freed up and now fully releases by itself, and the bias seems fairly even between the left and right.
It felt like the cable sticking, but I could be wrong… I guess there could be a yolk which divides the pull of the handbrake between the two wheels? Or maybe it’s just a weak return spring.
Let me know if you figure yours out; I’m sure mine will act up again soon…