FYI, this really isn't a Kibana question, it's about how to configure your Logstash pipeline. You're just using Kibana to inspect the problem. I can offer you some limited help, but if you need further advice, then the Logstash category is what you're looking for.
The filter alone doesn't ensure that the data is mapped the way you want it to be. If you have an elasticsearch output section in your pipeline configuration, you'll want to add a template and template_name as described in the Logstash docs: Elasticsearch output plugin | Logstash Reference [7.15] | Elastic. In the template you specify, make sure to map currentTimestamp to be a date field.