I think you should get interest if they paid you after April 15.
Doesn't work that way. If it did, then everyone would wait until Apr 15th to file. You don't start earning interest on your refund until 45 days "AFTER" the due date. With the normal tax filing deadline of Apr 15th, that would mean if the IRS did not pay your refund by June 1st, then they would have to pay you interest on the amount paid to you after June 1st.
But for the 2019 tax filing season the due date was changed to Jun 15th. So if you filed on or before Jun 15th, interest does not accumulate until *AFTER* August 1st of 2020.
Now if you actually read the link posted above concerning interest, it seems clear to me that there are those that will receive interest included with their refund, and there are those that will receive their interest as a physically separate payment from any refund due.
The website also makes it clear that the interest the IRS pays you is reportable on your 2020 tax return next year.
