For hydraulic shifted auto transmissions in general and not make specific, it is rare to have unrequested downshifts. If a clutch pack / band starts to slip when hot, the engine will flare and feel like a down shift but it really isn't.

Does anyone have a apply diagram for this trans? ( This is a grid that shows what is applied in what range )

There are two things that make the trans shift at the proper time, road speed and engine load. Road speed is measured by the governor and converted to variable hydraulic pressure. As road speed increases, the pressure goes up.

Engine load is measured by a cable or linkage attached to the throttle or a vacuum modulator referenced to the intake manifold. This also converts engine load into a variable pressure. ( This is sometimes called TV pressure, Throttle Valve Pressure ) Some transmissions have both using the linkage for kickdown.

Each "speed , gear" has a shift valve ( spool valve ). Governor pressure is on one side and TV pressure on the other. When governor pressure exceeds throttle pressure, the valve moves and the shift occurs.

There are a few valves in the mix for fine tuning, but the above is enough for a basic description.

So , to down shift, governor pressure must drop off ( car slows down ) or TV pressure to rise ( push harder on the gas ) . Even if the electric kickdown is stuck, the trans should up shift given enough RPM.

Any sort of "trans fix in a can" is more like "trans destruction in a can" . Put this in a trans that does not have friction material or piston rubber trouble, the frictions and rubber seals will be damaged.

A valve body leak / governor pressure loss of can cause a downshift but that isn't common. Installing presure gauges can give you some information.

The last question is did you buy this car to flip or keep?