As mentioned, The multi-select control works only with the template UI. A table with the 'Optimize Code' option unchecked, renders in the non-template mode. However, when the 'Optimize Code' option is enabled, the table changes to the 'Read Only' mode and allows the user to customize it. However, it does not provide the option to Add or Delete item. Hence, the multi-select control is not supported or does not work with the non-template table. This is the product limitation.
The feasible solution is to use the Repeating Dynamic Layout (RDL) instead of a table. Customize the RDL as per the requirement.