Reproduce the problem:
1. Add regular user to send message in acknowledgement configuration;
2. Login like Zabbix Administrator, select problem and click 'Bulk acknowledge' button;
3. Add some message and click 'Acknowledge'
4. Login like regular user and check Event details page
Result: Regular user in column 'User' see 'Inaccessible user' value, if it message was sent by Admin - OK
5. Open 'Action log' and check message to regular user

Result: Outgoing message contain text - Zabbix Administrator (Admin) acknowledged problem
Expected: Outgoing message contain text - Inaccessible user acknowledged problem