Because of multi-currency support, you have to Explicitly set the currency for that particular record while creating using SDK.
EntityReference currencyType = new EntityReference();
currencyType.Id = “(The Guid Of The Currency Type Goes Here)”;
currencyType.LogicalName = “transactioncurrency”;
entity.Attributes.Add(“transactioncurrencyid”,currencyType);
From CRM UI while creating, the lookup field transactioncurrencyid will be populated from user settings.
For the old records which got created already (when user settings was not set with default currency or in your case), you have to add that lookup into the form from Form editor, customize & publish. Then assign the currency in lookup for those records (may be bulk edit).