deltaExcludeParent header for delta link still includes root in response

Describe the bug
For delta links (https://learn.microsoft.com/en-us/graph/api/driveitem-delta?view=graph-rest-1.0&tabs=http), I use header: Prefer: deltaExcludeParent but it always returns the root alongside the driveItem that changed.

To Reproduce
Steps to reproduce the behavior:

1. Go to Graph Explorer
2. GET endpoint: https://graph.microsoft.com/v1.0/drives/{driveId}/root/delta with Prefer: deltaExcludeParent header
3. change permission on a file or folder in oneDrive
4. GET endpoint: the deltaLink from step 3 with Prefer: deltaExcludeParent header

Expected behavior
should only have 1 value in delta link response which is the changed file/folder. But current behavior returns 2 values, the changed file/folder and root (the drive)

Screenshots
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Desktop (please complete the following information):
 - OS: [e.g. iOS]
 - Browser [e.g. chrome, safari]
 - Version [e.g. 22]

Smartphone (please complete the following information):
 - Device: [e.g. iPhone6]
 - OS: [e.g. iOS8.1]
 - Browser [e.g. stock browser, safari]
 - Version [e.g. 22]

Additional context
Add any other context about the problem here.

Start by reproducing the request in Graph Explorer using the documented drive delta endpoint, the deltaLink, and the Prefer: deltaExcludeParent header. Compare both responses with the expected single changed file or folder; the payload names no repository file or test, so determining whether this belongs in Graph Explorer or the Microsoft Graph service is part of the investigation.