Tax liability on income tax refund. in which assessment year?

Take it as 'Income from other sources' in AY 19-20

You should consider the interest on income tax refund in the AY 19-20. In case of the non-resident the department would deduct the TDS at the rate of 31.2% (including cess at the rate of 4%). 

And also you can claim refund, if your taxes deducted is more than your tax liability.

IN A.Y. 2019-20 INTEREST INCOME ON I.T. REFUND 

PLEASE CHECK 26AS IN A.Y. 2019-20.

IF INCOME HAS BEEN SHOWN IN A.Y. 2019-20 BY INCOME TAX DEPARTMENT, YOU CAN CONSIDERED INCOME OF A.Y. 2019-20 AND CLAIM T.D.S. WHILE FILING INCOME TAX RETURN.

INCOME TAX DEPARTMENT ALSO SEND FROM 16A IN YOUR REGISTER MAIL ID.

Though the ITR was processed on March 24, he refund was credited in April. So will it fall in AY 2020-2021?