As far as I can tell meal voucher release is a Lead/Supervisor initiated item and not at all automated.

E.g. My last vouchered flight (in February) we deplaned for maintenance (the first plan was to change every wheel on the plane before ultimately becoming a plane change) and the supervisor* at the podium (in ORD) told the GA as I was walking off the jetway something along the lines of "I'm going to send the vouchers" while doing something on her MAP and less than a minute later I had a text with a voucher from UA. At that point I think the posted delay was somewhere in the 60-90 minute range.

*- At least by default if they're in plain clothes I interpret their role as supervisor-or-higher in the hierarchy; while leads and agents wear normal uniforms