You have created a code list restriction (CLR) for an object. The CLR default function works fine when you create the object directly, but it does not work when you convert the document, for example, if you convert a lead to an opportunity.
SAP Cloud for Customer
There is a limitation of the application on scenarios that are not standard create. The Code List Restriction defaulting functionality will only work on simple create scenarios (e.g. when you click Create new Account, Opportunity, etc.), but not on specific scenarios like conversion of documents.
An alternative option may be to check the SAP Cloud for Customer Customer Influence to submit an idea:
Convert, Code List Restriction, CLR, Default Value , KBA , LOD-LE-RC-CLR , Code List Restriction , Product Enhancement