The IVA covers all debts that were declared by the debtor, which should have been all outstanding debts at the time.
At times debts are overlooked when the IVA is proposed and get added in, if they are small in relation to the declared debts, during the course of the arrangement.
As this debt was defaulted in March 2016, that default, along with the debt, should have disappeared from the credit report in March this year. When did you last check the credit report and how ( there is sometimes a time lag if using the likes of Clearscore or Credit Karma) ?
Also, as long as this debt hasn't been acknowledged since 2016, it should become statute barred at the 6 year mark as well.
I would have a word with the former IP -- it could be that this debt would be considered to have been captured by the IVA after all.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk