Please tell me if I have taken the same steps you did. I was unable to replicate the issue on my end.
1) Change the Section Width to % and I chose 30.
2) Saved the screen
3) Change the Background Color for 2 cells in a column.
I then went back to see if the Section Width changed from % to px. It did not, it was still set to %.
Can you see if you are still having the same issue on your end?