Documentation page for user provisioning states that only default LDAP server is checked when non existing, in Zabbix database, user is logging in.
This is because frontend do not have information on what LDAP server user is using, provisioning for first time will associate user to LDAP server which is default at that moment and if default will change frontend will use already saved user relation to specific LDAP server.
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This is not a constraint, but expected behavior. To enable the use of multiple LDAP servers for JIT provisioning, there are additional considerations - for example, a prioritized list of servers and the ability to toggle 'non-default LDAP only' on or off. This would be a very useful feature and definitely deserves a ZBXNEXT.