I’ve just tried this and get good results same as your before pics. My settings are same as yours, so not sure what’s going wrong for you. I’m on M1 Pro.
Figured it out (sorta). Before, I would decimate at 50% over and over until I got down to where I wanted to be and it kept its shape. Doing that now causes it to loose shape along the way until it gets really far off.
I took the slider and slid it to 1% and it decimated in one round and kept its shape like it did before.
Side note: when decimating at 50% over and over, I get patches of less-decimated areas. Almost like it’s sacrificing some areas shape to keep higher resolution in some areas. See vid: