Hi ,
If you are trying to simply add one additional suffix, DHCP Option 015 will work for your DHCP clients. If you’re trying to add more than one additional suffix, GPO should be the better alternative. Generally, we don't enable them at the same time.
Based on your situation, we recommend you only use GPO and remove the DNS Search Suffix in DHCP option 15. Then see if the issue is gone.
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Best Regards,
Candy