Has anyone implemented for an Insurance company?

 Yes, this is actually standard Salesforce behavior, not a bug. When converting a Lead into an existing Opportunity, the Lead's currency does override the Opportunity's currency by default regardless of field mapping settings, since currency isn't handled through the normal Map Lead Fields mechanism. The common workaround is using a before update trigger or flow on Opportunity to preserve the original currency when conversion happens, since declarative field mapping alone won't stop this override.