Currently When you have a monthly repeating task with a due date for every 10th of the month and a start day 2 days earlier the next task (repeating) will still use the same start date and duration will change from 2 days to 33 for example
Start dates does not work properly for repeating tasks. I don't recommend using it differently from due date on recurring tasks.
This is due to the machine that runs while making new tasks on recurring tasks. System runs that machine and behaves a bit differently on iOS and macOS.
It's not crucial for me, but would be nice if it would work as I described. Setting alarm to earlier date seems fine as well for me to achieve what I need.
Since Reminders doesn't take care of start date, it's a bit of a mess. Currently if you set it differently, start date will become same value with due date on iOS. On macOS, start date just stays on previous value. I've suggested Apple about changing behavior to the way like you've mentioned long ago.