Unexpected and strange MOSFET behavior
M2 is not on, but its body diode is forward biased.
Here's the actual scheme of an NMOS:



simulate this circuit &ndash; Schematic created using CircuitLab
In the above picture, D1 is the body diode of M1. When you turn the NMOS on, it shorts the body diode.
Let's look at M2 in your circuit:



simulate this circuit
M2 is off here, but the body diode is forward biased. So the current flows through R2 and the body diode. You'll see that Vb is the forward drop of M2's body diode.
Same thing applies in your second circuit. The current flows through R2, M2's body diode and R5. And the current is determined by
$$
\mathrm{I=\frac{(V1 - V_F)}{(R2+R5)}}
$$.