Security levels remind:
Performance - you can perform regular JQL search.
Strict - you can search for secure fields only by using "issue in search" functions.
Disabled - search is not possible.
Important: changing Security level always demands Jira re-index, otherwise changes will not be saved.
Secure Single/Multi List in Performance mode -> not supper secure suggest strict mode, reindex jira or at leaset project
Switching from Strict to Performance, reindex Jira or at least project, check global search setting, then setting for particular CF
search that you perform
secure field's permission scheme
searching mode settings: Manage apps → Secure Fields → Search Security Settings
secure field settings: Issues → Custom fields → pick needed secure field and press Edit
configuration of the field: Issues → Custom fields → pick needed secure field and press Configure