Why ISO Format Date & Time gets Converted into the Array of Numbers while doing API calls

Question

I am using LocalDateTime in spring boot application and i am storing date and time in ISO format in mongodb and it is also getting stored there but when I am making API calls to the end points of my application it is showing list of numbers instead of showing the date in ISO format you can also see

Here is the class

I am not getting why it is working like this way.