Manual mode throttle requirement in automatic gearboxes

When using an automatic gearbox in manual mode, the game seems to require a minimum throttle application (or minimum speed) before you're allowed to shift up, even with gearbox assistant turned off. I know for a fact that not all real life cars with torque converters have such a limitation, since there's really no need for it, and while I am fully aware that taking off in 2nd or 3rd gear in a vehicle with a clutchpack, especially dry clutches, will cause excessive wear and damage to the vehicle, the gearbox assistant exists to prevent you from causing the vehicle such damage and I have it turned off.
So, with the assistant turned off, why can't I shift into higher gears with the manual mode in automatic transmissions when I want to? I know this used to be possible in the game before. Is there some option somewhere that I missed? Can I change something in the code of the gearbox to let me do this? Or is this just the game trying to resemble IRL cars more by refusing to let you turn assists off completely?

 

This was changed in a patch. You now need to add this line to the 'vehicleController' section of a transmission if you want no auto-downshifting;
"autoDownShiftInM": false

However the gearbox must have an M mode, which can also be added to "automaticModes":"PRND21" <--just put an M in there, usually after D.

 

This was exactly the answer I was looking for. Thank you very much.