Electrical – Low voltage on homerun

I suspect you may have something called a multi-wire branch circuit, which uses one set of three wires (rather than two pairs of wires) to handle as much load as two separate circuits. Current electrical codes require that multi-wire branch circuits must be powered by a pair of breakers that are interlocked so that whenever either one is turned off or tripped the other will likewise be disconnected, but some installations lack that interlock. Yours may be one of them.

If a circuit is run as two wires from the panel to the load, then when the breaker trips everything in the circuit will be de-energized. Likewise if a multi-wire branch circuit is run as three wires and both breakers are tripped or switched off, everything will be de-energized. Unfortunately, if only one breaker trips, devices on the side of the circuit controlled by disconnected breaker may receive a generally-small amount of current through devices connected to the live breaker. While such currents would generally not be large enough to cause a fire unless they were sufficient to trip the breaker on the other side, they could still be lethal to someone touching the "disconnected" circuit. Worse, a variety of factors may cause the amount of available voltage and current on the "disconnected" circuit to change unexpectedly, so even if a voltage tester shows that a circuit is "dead" that doesn't mean it won't seemingly-spontaneously become lethally energized as soon as the tester is put away.

Multi-wire branch circuits are cheaper to install than individually-wired circuits, and are perfectly safe when the breakers controlling them are properly interlocked, but can be dangerous when such interlocks are not installed.

Your test shows you have hot coming from the breaker (hot to ground 120 volts) but you are loosing it on the neutral side. I would start at the breaker box again only this time looking at (volt meter test) the neutral wire. If it's good go to the first junction box and test there and so on.