Index name not recognize +YYYY.MM.dd some times

Hi,
When I try to create an index in logstash output using:

elasticsearch {
  hosts => "myelasticsearch:9200"
  index => "my_index-%{+YYYY.MM.dd}"
}

In the log of elasticsearch the name of the index is "my_index-" and not appear the date.

Any idea?

Thanks.

I am confused because

output {
 if "_grokparsefailure" not in [tags] {
   file {
     path => "./logs/%{[fields][app_id]}-grok-%{+YYYY.MM.dd}.log"
   }
 } else {
   file {
     path => "./logs/%{[fields][app_id]}-grokparsefailure-%{+YYYY.MM.dd}.log"
   }
 }
}

And this is the result:

[2020-03-19T12:11:32,785][INFO ][logstash.outputs.file    ][main] Opening file {:path=>"/home/elastic/logstash-7.5.2/logs/squid-grok-.log"}

[2020-03-19T12:11:42,502][INFO ][logstash.outputs.file    ][main] Opening file {:path=>"/home/elastic/logstash-7.5.2/logs/squid-grokparsefailure-2020.03.19.log"}
[2020-03-19T12:11:43,462][INFO ][logstash.outputs.file    ][main] Closing file /home/elastic/logstash-7.5.2/logs/squid-grok-.log
[2020-03-19T12:11:53,568][INFO ][logstash.outputs.file    ][main] Closing file /home/elastic/logstash-7.5.2/logs/squid-grokparsefailure-2020.03.19.log