It must be ok then. I don't know how is your circuit, but if you use a two stage amp in your loop, it can be unstable. Even an single stage amp can lead to instability in the closed loop, depending on the load.
Are you measuring the phase margin by using the transient analysis overshoot? With cadence software, it is easy to see the open loop gain and margin of a closed-loop. With ngspice, I don't know how to do that. If you decrease the time step and the oscillation grows, be extra careful.