From a comment in JRA-25915

Not being able to search for users is not the only problem, also: when the user is the value of a user custom field, this field just does not show anymore for relevant issues, and a message is logged: "Issue XYZ-5 has an invalid value 'someuser' stored in the field 'User Custom Field'"

Note: This does not apply only the Active Directory, I've tested with OpenLDAP as well, but it must be a problem for all LDAPs.