Hello Dhruvin Moradiya,
Thanks for posting!
I tried to replicate the same scenario from my end, and I am able to face the same issue. As per my analysis, Delta only works with root folder but not with sub-folders.
Since this Feature/Functionality is presently not available, you can submit a feature request idea using this support link, which will be monitored by Microsoft team and make the enhancements to Microsoft Graph APIs. I will also upvote for you.
Hope this helps.
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