Why does triode voltage drop change as a function of grid voltage?
@jonk may speak harshly to me for over simplifying the model [ :-) :-( ] but the following may help.
Consider the triode as a voltage variable resistor = Rt.
Variation of resistance with voltage is non linear but that does not affect the model.
Below
Vg = applied grid voltage   (Vgrid)
Vt = triode anode-to-cathode voltage
Rt = triode effective resistance
It = Anode current
Rl = load resistor = anode resistor
Starting with Vgrid = 0 and then applying an increasingly NEGATIVE voltage to the grid then Rt increases. For an anode resistor Ra and a supply voltage Vs the triode anode Voltage Vt is
Vt = Vs x Rt / (Rt + Ra).
As Vg goes increasingly negative Rt increases and so Vt increases - because Rt becomes larger compared to Ra so drops a larger Voltage.
Similarly -
It = Vs / (Rt + Rl)
So, as Rt increases with negative-increasing Vg, It will fall.
Which is what you see.