Hi,

If you are using multiselect control in modal dialog or table layout it is not supported as per below explanation:

Below is the difference in rendering between templatized and non-templatized control: 
Templatized control renders on client side. On screen load the metadata is send by the server later on the specific content is getting load. In non-template control, the whole data is getting loaded on the server side and the result is shown on the screen. 
In the multiselectcontrol on the grid the section that is getting loaded is non-template and the multiselect control is templatized control. On screen load, the section getting loads the whole script with data whereas the multiselect control loads only the metadata and on second refresh it loads the data related to that previously loaded metadata. 
This is the reason why the template control is not getting loaded in the non-template section. 
Advanced parameter sections are rendering as non-template causing new controls like multiselect not to render. 

Hope this helps you.

Thanks,