This is a bug in the value suggestions endpoint of the filter manager, where we are using this line of code to load the index pattern:

const indexPattern = await findIndexPatternById(context.core.savedObjects.client, index);

The bug manifests in the following way:

1. Create a test index with a multi-mapped field, which is the default for any text. So for example:

PUT test-index/_doc/1
{
  "value": "My text value is multi-mapped"
}

2. Create an index pattern for the above
3. Go to Discover and search for value.keyword in the filter manager. Nothing will be suggested because it fails to load the index pattern field cache.

I think the only way to truly test this issue is to remove the field cache from all of the functional test archives. For example, I only ran into this problem because I removed the field cache from test/functional/fixtures/es_archiver/discover/data.json

To test this functionally, grep for discover filter editor