Rolling forward while in reverse gear

Occasionally for the last year when in Reverse my Prius' brakes will feel hard as rock and will only depress an inch or so. I depress the brake and within a literal inch it's solid and will not depress any more. I can still use the brake and reverse it's just jerky and awkward because I only have an inch to work with.. it's like they lock up or something? Well today same thing happened while in reverse but instead of the car actually reversing like normal when it has this hard brake problem it actually started rolling forwards while in reverse!! it was as if it was in neutral because touching the acceleration pedal did nothing.. which made me think of neutral cause it does the same thing, but the dash read reverse... all while I'm dealing with a hard brake. I have come to overlook it because I cannot mimic it and figured it was a possible regenerative/electric braking thing and seems fairly common among people here, but today was spooky because it rolled forward out of the blue..
Did the gear just slip even though it still showed reverse? Anyone have any ideas for me what is wrong so I can be more clear with the dealer?

 

That makes sense. It continued to read R even as it rolled forward, so I'm guessing it just didn't stick and slipped into N somehow and did not tell me.