Sorting using date field is fetching wrong records

I have an issue with the sorting of date fields in elasticsearch.
I have a date field as updateDate, When I try to sort based on this field , It is fetching me inappropriate records.
Like if i have records from January to September, at times , when i sort and fetch the records , instead of january i get the records of april month at the top.

Facing this issue when we have many records (more than 10,000 records)
This is the builder generated :
"sort" : [ {
"updateDate" : {
"order" : "desc"
} ]

This is the mapping used:
"updateDate" : {
"type" : "date",
"format" : "yyyy-MM-dd HH:mm:ss"