The procedure (which is seldom followed, IME), is they're supposed to solicit for volunteers to downgrade for a refund in upgrade method + the voucher.
If no volunteers, then downgrade in reverse upgrade order. An F award ticket is considered a paid F ticket. They are supposed to downgrade an upgrader first before downgrading a paid F ticket. If there were no upgraders, then yours would be the lowest paid F ticket, presumably.
So, based on the information provided (that there were upgraders), it appears that this downgrade situation, like most other UA downgrade situations, was mishandled.
At least you got the voucher, though. IME, they usually don't know about the voucher and you have to push for it.