Soft-Starter PWM Noise
Per the OAD-5 datasheet:


  The OAC5 output module is used for controlling or switching AC loads
  of 12-140 VAC. Each module provides up to 4000 volts (transient) of
  optical isolation between the field devices and the control logic.
  This module is equivalent to a single pole, single throw, normally
  open contact (FORM A, SPST-NO, Make). All AC output modules feature
  zero voltage turn-on and zero current turn-off.


The OAC5 is made for switching AC in a graceful way (at voltage and current zero crossings) and is not designed to support PWM.  As a result, the OAC5 is switching on when your PWM output is high AND the AC waveform has a zero voltage crossing, and the OAC5 is switching off when your PWM output is low AND the AC waveform has a zero current crossing.  The result will appear near-random, I'd expect.

It seems to me that what you probably want to do is phase control on the AC side of the rectifier or PWM via a FET on the DC side of the rectifier to achieve soft start.