LWC | @wire function call returns [object Object]

Hello I started learning Lwc, I have problem with chained @wire function call, which always returns [object Object] instead of field values.

Controller

public class LastMonthStatementComparatorController {

    @AuraEnabled(cacheable=true)
    public static Bank_Statement__c getPreviousMonthStatement(String statementName) {
        return [SELECT Name, Income__c, Outcome__c FROM Bank_Statement__c WHERE Name = :statementName];
    }
}

@wire annotaded variable which works:

    @wire(getRecord, { recordId: '$recordId', fields: [BS_INCOME, BS_OUTCOME, BS_NAME] }) currentBs({err,data}) {
        if(data) {
           this.previousBsName = this.parsePreviousBsName(getFieldValue(data, BS_NAME));
           this.currentData = data;
           console.log(data)
        } else if(err) {
            console.error(err);
        }
    }

second @wire that is executed after first one

    @wire(getPreviousMonthStatement, { statementName: '$previousBsName' }) previousBs({err,data}) {
        if(data) {
            this.previousData = data; //here it works, json is retrieved successfully.
        } else if(err) {
            console.log(err) 
        }
    }

getters

    get currentIncome() {                                   ====> returns valid value
        return getFieldValue(this.currentData, BS_INCOME);
    }

    get previousIncome() {                                  ====> returns [object Object]
        console.log(this.previousData)                      ===> it returns valid JSON
        return getFieldValue(this.previousData, BS_INCOME); ===> it returns [object Object]
    }

HTML

<template>
        <lightning-card  title='test'>
            <p class="slds-m-top_medium">{currentIncome}</p>
            <p class="slds-m-top_medium">{previousIncome}</p>
            <p slot="footer">Card Footer</p>
        </lightning-card>
    </template>
</template>

Where's the problem?