This is not an error in PLECS but simply the response of your circuit. This forum is generally intended for PLECS questions and not general power electronics design issues, but an explanation is below.
With the LC filter at the output, the harmonics in the DC voltage are due to the combined effect of the rectifier input and inverter output harmonics.
With only a resistive load the behavior arises from the controller and modulator. The controller is attempting to regulate the output voltage which toggles between +/-Vdc every switching event since there is no output filtering. This, along with the feedforward in your circuit results in the modulation index toggling between very high and very low values every switching cycle. For some portions of the cycle this results in an effective 0.5 duty cycle. Your controller isn't properly handling this condition, although please consider if a purley resistive load without any line inductance is reasonable.