Well I hope after this time you have got answer but If not then actually your native approach is the first one I tried myself first for my update of a field in a model. But then tried the below slightly different approach
the mistakeI made earlier is that for some reason we need to provide emp in emp.emp_total_allocation , i.e. we have to fetch the collection for which we want to update.
let emp = await strapi
.query("employee")
.findOne({ id: data.alloc_emp });