Unplugging Ethernet Cable Leads an Abnormal Power Supply Fail
If you suspect the ferrite bead:
You can use a spice model for your ferrite bead and simulate it with your actual caps values on both ends, see if that gives a spike in the frequency response. Ferrite beads become resistors at HF, but at low frequency they're inductors, so with low-ESR ceramic caps you can get nasty ringing and spikes at some tens of kHz. If Wurth doesn't give ferrite beads, you can try to find an equivalent model from Murata.
You can try probing the power supply rails with a scope and set the trigger to slightly above or below normal voltage, see if the LC filter formed by the ferrite and the caps rings and makes voltage spikes/dips.
No need to make the &quot;destructive&quot; test by unplugging the port, if you see suspicious ringing with the PHY working then that's a clue.
Now you're using a 1000 ohm ferrite bead which means it must have quite a bit of inductance (should be about 5µH from the 30 ohm impedance at 1MHz), so the question is, does it saturate at the current you're using? If it does, you will get ringing when the device stops drawing current, but you won't when it turns on because the inductance will be lower due to saturation. So the above test wouldn't work.
If you don't want to do it on your board you can assemble a test circuit with the same ferrite bead and caps on both sides, power it from one side with a bench supply and instead of the PHY you can use something like a transistor controlled by a square wave from a function generator to switch on/off a resistor that will draw as much current as the PHY does.
If that's the problem, solution is to either use a different ferrite bead with less impedance, or add mode damping, for example more caps with higher ESR.