Thank you for posting this in Microsoft Q&A.
As I understand you have Global Admin and User Admin roles assigned accessing a customer Tenant in GDAP and PIM. You are unable to make changes to the dynamic membership rules for groups.
This is by design, if you have granular delegated admin privileges (GDAP), then Administration of dynamic membership rules is not supported.
The same is mentioned in below article,
If you want to change this behavior, or if you want any additional features in Azure, then you can submit feedback in our Azure feedback portal. This portal is directly managed by our PM's.
https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789
Let me know if you have any further questions.
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