1
You are missing PermissionsString in your code.
As per the reference you provided, you can add any permission type from the below list based on your requirement:
FullMask
EmptyMask
ManagePermissions
ManageSubwebs
ManageWeb
AddAndCustomizePages
BrowseDirectories
ViewPages
ManageAlerts
Read more about SharePoint Base Permissions at SPBasePermissions ...
1
The screenshot you provided looks like an external sharing link, but based on my testing, I cannot reproduce your problem.
If possible, please provide more information on setting permissions for these folders step by step.
How to Set Unique Permissions in SharePoint:
https://oit.unr.edu/services-and-support/data-storage/sharepoint/sharepoint-shared-documents/...
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