For LeadQualifyRequest, there is an attribute named "OpportunityCurrencyId" which should be explicitly set in the request. This is not the case for other mappings (Lead to Contact & Lead to Account).

Logically ,there should be the default behavior for this – if we are not setting currency in LeadQualifyRequest then it should consider the OOB currency relationship mapping which is not happening.

Reference doc:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.crm.sdk.messages.qualifyleadrequest?view=dynamics-general-ce-9

https://docs.microsoft.com/en-us/dotnet/api/microsoft.crm.sdk.messages.qualifyleadrequest.opportunitycurrencyid?view=dynamics-general-ce-9#Microsoft_Crm_Sdk_Messages_QualifyLeadRequest_OpportunityCurrencyId