In Salesforce Lightning Experience, custom fields or standard fields added to an object's page layout (such as the Task object) may not appear in Quick Actions — for example, the New Task global action. This is a known behavior difference from Salesforce Classic.
Quick Actions in Lightning Experience use a separate action layout called the Global Action Layout, which is distinct from the standard object page layout. Adding a field to the object's page layout does not automatically add it to the Global Action Layout. Fields must be added explicitly to the Global Action Layout in Setup.
To make a standard field or custom field visible in the Quick Action in Salesforce Lightning Experience, add it to the Global Action Layout by following the steps below.
Note: This behavior applies to all Global Actions fields in Lightning Experience. Each Quick Action has its own independent layout that must be configured separately from the object's page layout.
Action Layout Editor
Quick Actions Implementation Guide
Create Object-Specific Quick Actions
Set Predefined Field Values for Quick Action Fields
Object-Specific versus Global Actions
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