Looks very strange because the data sheet gives a load & line reglation of 4 % max ( summed ) that means a maximum drop of 0,2 Vdc.
Are you sure about size and lenght of suppy cables ( 5 V side ) ?
Remeber that copper has a resistance of 0,019 Ohm * meter/ square mm, that means at 8 A ( hope your model is 229075K as highlighted ) every meter of copper with a section of 1 square mm gives a voltage drop of 0,15 V ( take also care that 1 meter copper means 0,5 metrr for + terminal AND 0,5 meter for return terminal ) and this could be the problem.
Let us know the results.
Mandi