Do you mean when the mast is FULLY EXTENDED (all the way to the top) or just if the forks are off the ground?
Does it act the same way when the forks are unladen, or only when a load is on them?
The simple approach to this is to ask...Is the hydraulic tank filled to its correct level?
If the hydraulic tank is not filled to its correct level, it is possible that the pump is running out of oil after filling the lift cylinders to the point there is not enough oil left to operate tilt back.
But, if that were the case, the mast is likely not being lifted to its fully extended height at all.
Another possibility is that the truck might have the Toyota SAS system on it. If that is the case, it just got more complicated.
Look on the mast where the large hydraulic hose coming from the truck connects to a squarish shaped valve.......does this valve have a sensor with a wire attached to it?
If there is a wire connecting to a sensor on that valve the truck may possibly have the Toyota SAS system on it.
If so, you may have to have a Toyota tech check out the truck because the Toyota SAS system controls the hydraulic functions of the mast and steering axle under certain conditions.