A View rule with the field "View" set to Default view will remove the "New" UI Action in related links.
Steps to Reproduce
1. In the Incident table, add "Incident Task" in the related links.
2. Notice the "New" button is available for users validating the relevant ACL.
3. Create a new view rule on the Incident table, setting the "View" field to Default view.
4. On an Incident form, open the settings by clicking the top-right gear icon.2
5. Set the "Related list loading" in Form section to "On-Demand".
6. Open an Incident.
7. Click on "Load Related List" on the bottom of the form. When the related list is loaded, notice the "New" button has disappeared from the Incident Tasks.
After carefully considering the severity and frequency of the issue, and risk of attempting a fix, it has been decided to not address this issue in any current or future releases. We do not make this decision lightly, and we apologize for any inconvenience.
As a workaround, change the "View" field in the view rule to "Default".
Related Problem: PRB746901