“Adventures required for the Arrow of Light rank must wait until the Webelos rank has been earned.”
4.1.1.4
This would include the one required elective adventure. It doesn’t say “except electives” anywhere in the rules. It means required in terms of requirements, not required in terms of only the four adventures.
I have an Arrow of Light requirement show up as completed in Nov 2021, they earned Webelos in May 2022.
So according to the guide to advancement Scoutbook should only be auto-assigning adventures for Arrow Light that are earned after the date Webelos rank was completed. It doesn’t need to wait for the program year to roll over, just to earn Webelos.