This happened to me too as a result of the same problem you're experiencing - download completion regressing to 0% or below. I had the exact same number show up on my download progress bar, my guess is it's a coding thing. When the progress bar goes negative (for whatever absurd reason), the 64-bit binary value probably flipped back full circle from its minimum value (all zeroes) to its maximum value (all ones). I checked this in excel, and that math seems to add up.
TL/DR, it's just another symptom of the weird download completion regression stuff we've been experiencing.
