I've this custum field with definition:

Custom Field ID: 1 Custom Field Name: Required Is Active: Yes Custom Field Type: Textbox Dropdown Listbox Listbox Checkbox Date Screen Order:   1-100 Screen Label: Screen Tooltip: Maximum length:   1-7000 Number of columns:   1-100 Number of rows:   1-100 Required: Yes Regular Expression Validation: Default field value (on-screen): Can multi-select: Yes Lookup table name: Lookup key field name: Lookup description field name: Lookup sort field name:
Will default to description if left blank Filter by project id: Yes Is visible on issue create: Yes Is visible on issue edit: Yes Is visible on issue view: Yes Projects: M001 - Modulo programmi di base per la contabilita' or... M002 - Gestione della produzione

The issue filter, search additional information on this custom field "Codice cliente" without LIKE pattern %mystring%.

If custom field value on issue is "11111;22222" , and I search 11111 no issue is found.