Issue Summary

This is reproducible on Data Center: yes

Steps to Reproduce

1. Create an Issue
2. Crate two Epics: Epic1 & Epic2
3. Edit the Issue and try to add one of the Epics, start typing "Epic"

Expected Results

Expected behavior, locally tested in Jira version 8.17.0:

Also, making a REST call, returns the expected values:

<base-URL>/rest/greenhopper/1.0/epics?searchQuery=Epic&projectKey=AA&maxResults=100

As it is shown, it seems that it was working as expected in old versions like 8.17.0

Actual Results

Faulty behavior:

<base-URL>/rest/greenhopper/1.0/epics?searchQuery=Epic&projectKey=AA&maxResults=100

Workaround

Workaround 1

Typing the full Epic Name will show the result:

Workaround 2

From our KB:
Performance updates to Jira epic picker

If you want to revert to the old mechanism, you need to enable the following dark feature: com.atlassian.jira.agile.darkfeature.legacy.epic.picker. For details, see How to manage dark features in Jira.

1. In a web browser open URL : <BASE_URL>/secure/admin/SiteDarkFeatures!default.jspa
2. To enable a feature flag, type com.atlassian.jira.agile.darkfeature.legacy.epic.picker.enabled in the Enable dark feature text area and click the Add button.
3. Go to your Issue and check if it is showing any match when staring to type the Epic name.