@Mahesh Chandak Thanks for your patience! I am able to reproduce this issues inhouse this seems to be a Bug. I believe your APIM instance is in VNET correct me if I am wrong.
Please try removing your service from VNET, perform the desired operations such as adding custom domain, enabling Request client certificate and add service back to VNET. This is a preview SKU and is expected to have some problems.
Do let me know if you have any queries.
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