On the BB sever there was a car factory, next to but outside a medium-sized town. It produced quite successfully for many years and it grew larger several times. After one such growth event it had 48,000 open jobs but stopped producing. Several players supplied bus and train routes to nearby cities, but the number of employed never rose about a few dozen, and those always disappeared quickly. It never again made another car and finally shut down.
I have got exactly the same issue on one of my maps. A car factory has 48,000 open jobs and no matter how fast I ship in commuters, I can never reduce the open jobs by more than a few dozen. It has never produced a car. This car factory was present from the start of a new map created in 1945, so the upgrade and field growth aspects of your situation weren't present in mine.
Links to the savegame and pakset are in this thread (https://forum.simutrans.com/index.php/topic,23834.new.html#new) if anyone wants to see it; it's in the town of Pakenham.