There are 3 tables in Shift sheduling, downshift prevention, enter last row 250 in all 3 and you will be able downshift from 6th to 5th at any speed bellow 250.
Entering higher values than 250 does nothing and it's not resolved. No problem to pull 5th to max speed you can.
Could be not all EB calibrations has this tables added, i requested sometime to add in EB and v6.