In Origin2025b, new two graph and click Graph: Merge Graph Window: Open dialog
Keep the default setting and click OK
Start Origin2026, new two graph and click Graph: Merge Graph Window: Open dialog
Keep the setting (Last used theme created in 2025b) and click OK
==> Variable value is not in predefined list, VarName = arrange, VarValue = 2
Fixed in Origin 2026 SR1