Elastic Search ILM implementation - Upsert issue

Elastic Stack Elasticsearch
1

We have 2 indices - "event" and "event-001"
"event-001" is created by using ILM

Both the indices have the same alias "event_"
It is used to fetch and update the data in the above 2 indices

Query:
GET event_/alias
Result:
{
"event-001" : {
"aliases" : {
"event" : {
"is_write_index" : true
}
}
},
"event" : {
"aliases" : {
"event_" : { }
}
}
}

Problem scenario:
When there's a record in the "event" index with "id" as ab1 and it is not present "event-001"
I try to update the record using the alias index as below.
Query:
POST event/_update/ab1

{
"doc": {
"city": "vijayawada"
},
"doc_as_upsert": true
}

Instead of updating in the "event" index, it is adding a new entry in the newly created "event-001" index.

How can I update an index where a record is present?

Can you please help on this.

2

Using ILM automatic rollover, write_index is automaticaly set to the new index. You can use Update by query API to update docs but upsert seems difficult.