@Keerthi0555 : If you save the CF model after selecting only 1 model in this field, it may store "fragmentmodelreference" as a String property. However, when you select multiple values, it stores all of them but the property type still remains String. I checked this in CRX and can see it stores multiple values.
What you should check is if it lists all the content-fragment models creating using all of these models when you create a content-fragment using this model.
thanks.