A multiple Update UI Action performs the correct update functionality (the desired Action) but gives an incorrect message as below:
(1)Multiple update did not update 2 records
(2)Multiple update complete, total records updated: 0

The issue is reproducible only in Madrid and not in any of previous releases.

In any Madrid instance

1. Navigate to System UI > UI Action
2. Create a new UI Action with following information:
-Name: UpdateNotes
-Table: Incident [incident]
-Action name: update_notes
-Active, Show insert, Show update, Client, List choice: Select these check boxes.
-OnClick: showQuickForm('UpdateNotes','update_it')
3. Click Submit
4. Create another UI Action named "Update_it" with below information
-Name: update_it
-Action name: update_it
-Script:
current.comments = "Update: Action executed";
current.update();
5. Click Submit
6. Navigate to incident.list
7. Select multiple records from list view
8. Select UpdateNotes in the action choice list below the incident list of records
9. On the dialog box that appears on screen update any field say short description
10. Click on the 'Update' button on showQuickForm dialog box
11. Observe the message shown at the top

Actual Behavior: Message displays as
(1)Multiple update did not update 2 records
(2)Multiple update complete, total records updated: 0

Expected Behavior: Message should display as
Multiple Update complete after updating 2 records

The UI Action for updating the comment does execute successfully but it gives an incorrect message stating system did not update the data.

This issue reported here was caused as a result of incorrect documentation in the link below which has been corrected.  

• Create a UI action to close multiple incidents

Previously, step 6 in the Procedure section of the docs suggested to create a new Business Rule instead of a new UI Action which caused this issue.

The docs link above has been corrected and the Multiple Update UI Action should work as expected by following the updated steps in the documentation.