This does happen with some hypos, not all I find, but some. It takes ages, sometimes, for my blood sugar to rise, and then it starts rising rapidly, and goes up in an almost straight line.
I find with the libre I can anticipate this a bit, because it's not good to spend to long hypo, so if I know I've eaten too much, relative to insulin on board, to correct it, I'll take a little bit of insulin after my blood sugar has reentered the 4s. Then you have to watch it carefully just in case, but usually the insulin ends up chasing the rapid rise, but it does stop it going way too high, and gradually brings it down. Chasing the upward rise after a hypo is annoying, but you'll get better at it.
Don't be angry with yourself about it, though. A 20 is horrible, but you aren't going to stay there long, and being hypo for too long isn't good either.