What order the FA takes meal orders in is completely random and up to the FA. There may be a policy written down somewhere but in practice it's total anarchy.
I have had total adhearance to FEBO for the past few years unless there was a mistake. I always pick my seat location based on the flight number. Last week, DEN-ATL on an even numbered flight, I was in 2C. The FA started taking orders from the back, when she had finished with row 5, I got her attention and told her that I thought the orders should be taken from the front on this flight. She looked at her order sheet and apologized saying she was mistaken. The lady next to me thanked me as she also picks her seat based on the flight number.