Designing 80V CMOS Circuit
You've missed an important point. The data sheet specifies the gate-source voltage as a maximum of 20 volts. Look at your circuit. Pretend, just for the moment, that the 20k resistor were zero ohms. Then, with a 0 volt input, what would the gate-source voltage on the p-type be? Yup - 80 volts.

With the n-type off, the full power supply voltage is applied to the p-type gate, since there is no current flowing in the 20k. At 24 volts, this means that the p-type was always on, but the voltage was still something the p-type could handle (violating a max rating does not guarantee failure - it just means the device is no longer guaranteed to work. There's a difference.). The presence of the 20k meant that when the n-type turned on you didn't get shoot-through and destroy one or both transistors, so the output sort of worked.

When you went to 80 volts, though, it was more than the p-types could handle. Their gate junctions were destroyed and they failed open.

You say you've tried several different MOSFETs. I think that, if you go back and try them at 24 volts, you've killed them all. 

You're also lucky you didn't destroy your PWM source.