Voltage drop on string of constant-power loads: Simulation issue?
Constant power loads are fundamentally problematic.

\$P = IV\$ rearranges to \$ I = \frac{P}{V}\$, so as V tends to zero I tends to infinity.

In other words to get a truly constant power load "off the ground" requires an infinite current. In practice loads are not truly constant power, but the current required to get them "off the ground" can be much higher than the normal operating current.

I'm not familiar with the internals of LTspice but I would guess it uses some form of iterative approach, depending on where it starts from the loads may or may not come "off the ground".

To make a useful overall model you need a better model of your loads, over what range of voltages are they constant power? what is their behaviour at voltages outside of that range. Unfortunately I don't know if there is a way to build custom models in LTspice.