This happens when a Sales or Purchase Account Code has been set on the individual Price List Item under Settings > Price List & Kits.
Any Account Codes entered on individual Price List Items overrides the defaults set under Settings > Accounting System and any Account Codes entered on the Invoice lines for that item. Essentially the Account Code becomes hard coded for that Item.
To remedy this, you need to either remove or change the Account Code set on the individual Price List Item/s under Settings > Price List & Kits.
If the Account Codes on the Price List Item are left blank, Tradify uses the default Account Codes specified under Settings > Accounting System.