Service: Custom Label for Subject Appears in Case Original Message

Issue: 
When sending an email from a Service [Case] in Lightning Experience using the Chatter component, the "Subject" field name within the [Original Message] section may display a different label. This occurs because the [Original Message] uses the label defined for the [Subject] field on the [Activity] object. If this label has been modified, the custom name will appear instead of the standard "Subject".

Solution: 
To resolve this, rename the [Subject] field label for the [Activity] object via [Rename Tabs and Labels].

1. Click the gear icon and select [Setup].
2. Enter "Tabs" in the Quick Find box and select [Rename Tabs and Labels].
3. Click [Edit] next to [Activities].
4. Click [Next] on Step 1.
5. In Step 2, change the label for [Subject] to the desired name (e.g., Subject).
6. Click [Save].

Note: 
Changing this field name will affect the display label for the [Subject] field on all [Event] and [Task] records globally.