[SalesForce] LWC: Call an Apex Method Imperatively

I have a simple button handle click function which calls the apex method and pass the params. However, if I pass the @track variable as param, then apex method is not getting called. If I pass the static value it is working fine. not sure what I am missing.

Javascript Method: (Working)

  handleClick(){
            window.console.log('inside button click: '+this.selectedOptionsList);
            const fLst = 'type';
            window.console.log('fLst: '+fLst);
            fetchPermissions({ sObjectName: 'Account', fieldList: fLst})
                .then(result => {
                    //this.contacts = result;
                    window.console.log(JSON.stringify(result));
                })
                .catch(error => {
                    this.error = error;
                });

        }

Console Log: (Working)

Javascript Method: (Not Working)

handleClick(){
        window.console.log('inside button click: '+this.selectedOptionsList);
        const fLst = 'type';
        window.console.log('fLst: '+fLst);
        fetchPermissions({ sObjectName: 'Account', fieldList: this.selectedOptionsList})
            .then(result => {
                //this.contacts = result;
                window.console.log(JSON.stringify(result));
            })
            .catch(error => {
                this.error = error;
            });

    }

Console Log: (Not Working)

Dual-list box onchange Event:

handleChange(event) {
    // Get the list of the "value" attribute on all the selected options
    this.selectedOptionsList = event.detail.value;
    window.console.log(`Options selected: ${this.selectedOptionsList}`);
    }

Simple APEX method:

@AuraEnabled(cacheable=true)    
    public static String getPermissions(String sObjectName, String fieldList){


        return fieldList;
    }

If you see in the log, both @track variable value and cont variable value are the same. However, I don't get the result if I pass the @track variable as you see in the log.