Is this opto input current sized wrong and can it have such symptoms?
Yes, this definitely could be the reason.

You forgot to include the opto's LED Vf in your calc.  Vf typ. is 1.3 V, 1.5 V max.  That is a worst case current of 7.4 mA, worse than you thought.  The secondary current is around 5.5 mA, way over 20%.  The simple fix is to change to an opto with higher CTR.