Username auto-fill does not work after the first user is selected in User Option macro.
The user search or auto-fill only works for the first user. After adding a comma and typing the second user's name, the "Not found" error is shown.
Confluence or JIRA version
Insert the User Options macro > Click Edit.
In the User(s) field, insert the first user followed by a comma and insert the second user.
The auto-fill will work for the first user. For the second user onwards, an error will be shown and the auto-fill will stop working.
The comma value is ignored, the field takes the input of the usernames as an entire string when searching.
After inserting the first username, input the comma value and type the second username onwards manually.
The error "Not found" will continue to show, you can ignore this.