SheetJS: write method returns blank sheets when sending data to FormStack

Question

I am making a API call to FormStack which has a xlsx template I am using to pass some data to.

The idea is I am sending different data to this template, and each time I do that operation, that data will populate a new sheet. At the end I should have a excel document with n amount of sheets. e.g. [ { sheet-1-data },{ sheet-2-data } ] => spits out excel doc with two sheets with each sheet styled/based on template living in FormStack.

My problem is while it generates a file generated with two sheets, each tab is blank.

I am using Nest.js, And this is my service which is processing the Buffer.

@Injectable()
export class DSDocExcelProcessorService {
    constructor(
        private readonly logService: DSLogService,
        private readonly auditService: AuditService
    ) {}
    async processExcelDocs(contentArr: { order: number; fileContent: Buffer }[] = [], docGenReq: DSDocGenerateRequestModel): Promise<Buffer> {
        try {
            this.logService.info(
                `Started processing Excel generation for Request Id ${DSRequestContextService.getCurrentReqId()}. Total Excel docs to be processed : ${
                    docGenReq.templates.length
                }`
            );
            const finalXLSX: WorkBook = await utils.book_new();
            let xlsx: Buffer;

            for await (xlsx of orderBy(contentArr, 'order').map((item) => item.fileContent)) {

                const workbook: WorkBook = read(xlsx, { type: 'buffer' });

                utils.book_append_sheet(finalXLSX, workbook);
            }
            this.logService.info(`Successfully uploaded file ${docGenReq.outputFile} to ${docGenReq.destinationBucket}`);

            return write(finalXLSX, { type: 'buffer', bookType: 'xlsx' });
        } catch (error) {
            this.logService.error(error);
            await this.auditService.updateRequestStatus(DSRequestContextService.getCurrentReqId(), EDSRequestStatus.ERROR, true, [error.message]);
            throw error;
        }
    }
}

Any thoughts as to why I am getting no template of data for it in each sheet?

Any help would be appreciated!