If a UI Action does not have an Action Name, it is not possible to override it for an extended table.
1- Login as administrator
2- Create a UI Action for Task table (you can use the existing 'Assign to me' UI Action, just enable Form Link to test the issue easier)
3- Override the UI Action for Incident table by following the steps: https://docs.servicenow.com/csh?topicname=t_OverrideOrRmvAUIActionForExtTbl.html&version=latest
(Name: Assign to me, Active: true, Form Link: true, List context menu: true, Show insert/update: true, Script: current.incident_state = 4; current.update();)
4- Go to any Incident record, click 'Assign to me'
Notice that the UI Action still points to the one created for Task table
(If you've enabled Form Link, right click to the UI Action and click 'Edit UI Action', you will be redirected to Task's UI Action)
Even adding the Task's UI Action to 'Overrides' field doesn't change the behaviour.
This is expected behaviour. As a workaround apply one of the following: