Searching based on custom header field in APM index

Hi Team,

I need to search based on a custom header field, say "context.request.headers.ua-device", I followed below procedure to make that field searchable by changing the index template.

But even after indexing the field (Exclamation mark corresponding to the field, under Discover, has disappeared), If I perform filtering based on this field it is not working.

I even tried changing index setting of the current APM index, settings.index.query.default_field, but not helping.

Kindly help me with this.

FYI, I'm using Elasticsearch-6.7.1, running on ubuntu -18.04 Bionic OS. Kindly look into this issue