Unexpected LTSPICE Output - Voltage Mode Buck Convertor
It's going to be a crappy switch mode converter design because you have the FET connected as a source follower: -



If FET has a gate threshold voltage of (say) 4 volts then you cannot get any voltage out of the source higher than 30V from your circuit. Also, with it in this configuration the rise and fall times of the FET are totally limited by the rise and fall times of the op-amp delivering the "square wave". As the op-amp has 2.5 volts per micro second slew rate, it makes for a pretty ineffective switcher.

The rise time for a 30V change is 12 us. Fall time is also 12 us. Given that you would want these times to be less than 5% of a cycle time, this makes the best effective switching frequency about 2 kHz.

Fix the design either by using a P channel device or use a proper driver chip. Also, be aware that the LT1677 has diode clamps across its inputs hence, if you have 4V on one input, the other input is constrained to levels between 3V and 5V - this might answer one of the issues mentioned in the question.