Ok, so TT did it wrong, and my previous fix did not fix that. I have changed it to work as in stock in r3935. Now to decide what to do with the harvester becoming invisible and not moving.
The problem is that the harvester is delivered (as when the "Unit Ready" text appears) more than 12 seconds after the (automatic) vehicle request. After 12 seconds, the airstrip discards all information about the unit it was constructing and reenables the vehicle purchase buttons in the PT. Because it resets, the airstrip does not finish the delivery of the vehicle, so that the harvester stands still, invisible, and no "Unit Ready" text appears.
The reason why this happens only for Harvesters is because they take longer to construct. Likely Westwood error'ed when lowering the delivery time and only applied it for the general case (any vehicle manually purchase) but forgot about Harvesters (automatically requested by the refinery), leaving the latter with a longer delay. Anyway, Harvesters take 8 and normal vehicles 5 seconds to deliver when the base is powered; and 16 and 10 respectively if not.
Additionally, because the cargo plane always takes at least 6 seconds to arrive at the red zone, normal vehicles are actually delivered after 6 seconds rather than 5 when power is on. So, summarizing:
Harvester: delivered ("unit ready") after 8 seconds from purchase if power is on, after 16 seconds otherwise (but becomes invisible and does not move because 16 > 12).
Any other vehicle: delivered ("unit ready") after 6 seconds from purchase if power is on, after 10 seconds otherwise.
12 seconds after a previous purchase, you can buy a new vehicle.
There are two problems due to the way this works:
1. The harvester becomes invisible and does not work.
2. If the power plant is down, Harvesters have too little time to drive off the red zone if another vehicle is purchased directly afterwards, so that they get killed and are repurchased; wasting Nod's time and hampering income. I think it sometimes happens even if base power is online.
I see three possible solutions:
1. Fix Harvesters to work and be visible, change nothing to the build times. That solves the first problem but leaves the second.
2. Change Harvester delivery time to match delivery time of other vehicles. Since all vehicles will be delayed by the same amount, this ensures that vehicles have 6 seconds to leave the strip, which should be sufficient for the Harvester. Harvester delivery times will change from 8 and 16 to 6 and 10 seconds when power is on and off respectively.
3. Change finish times (normally 12 in all scenarios) to depend on the actual vehicle delivery time. Then vehicle delivery times remain the same but time-between-vehicles increases. That means players have to wait longer after a Harvester is being purchased, which may be a disadvantage.
I think I like solution 2 most, because I see no reason why Harvesters should take longer than other vehicles, and I don't think it would be good to make players wait longer for Harvester purchases.