[Discover] "Add Filter" treats field contents case insensitive, but does filter case sensitive

[rsdrakh]

Kibana version: 8.4.1

Elasticsearch version: 8.4.1

Server OS version: Ubuntu 22.04 LTS

Browser version: Any

Browser OS version: Any

Original install method (e.g. download page, yum, from source, etc.): Any

Describe the bug:
When you have a field that contains the same information in different case notation (Example: host.name : Node01, node01, NODE01) between different indices (happens out of the box with different integrations), and you need to set a filter for that host.name, you need to add a filter like "field: host.name, operator: is one of" in Discovery. While picking the different case sensitive options, Kibana shows all 3 values in the list of host.name contents, but as soon as you add node01 as a value, Node01 and NODE01 disappears from the list and typing it says "node01 has already been added". The resulting filter, however, is case sensitive, so NODE01 and Node01 will not be shown in the results.
This is highly inconsistent.

Steps to reproduce:

1. Have an index with 3 different documents, containing "host.name" with any of "node01", "Node01", "NODE01"
2. Add Filter in Discovery picking "host.name" as field, "is one of" as operator, and try to pick all 3 variants as value from the list.
3. save filter

Expected behavior:
If Filter UI value picker says "already picked" then the resulting filter should also be case insensitive
Screenshots (if relevant):

Errors in browser console (if relevant):
none

Provide logs and/or server output (if relevant):
not necessary

Any additional context:
This is an actual customer complaining.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[Dosant]

Thanks for reporting this!

this looks like a duplicate of #49450, but looks like that was closed by mistake, so keeping this one open.

The bug is coming from the UI library elastic/eui#1997. As soon as it is fixed, we can improve this in Kibana filters

[jughosta]

@stratoula @lukasolson
Eui now supports isCaseSensitive prop for EuiComboBox elastic/eui#6268 and we can fix it for Unified Search.