Using WS_DELIVERY_UPDATE_2 in a Loop

Hi All, I’ve stumbled upon a strange behaviour when calling WS_DELIVERY_UPDATE_2 in a loop. Given 2 valid vbeln, when being executed individually both gave success response and VL33N showed an intended result. But when using it in a loop, somehow the 2nd document gave error message instead, which consists of HU of the 1st document, even though no HU is provided in the parameter.