I am seeing similar issue for V2 DXAPI as well when we invoke Assignment Submit API PATCH /assignments/{assignmentID}/actions/{actionID} with a new PageList Property, we are currently using Pega 24.1.1 

One more Observation Not sure if its relevant but When we Send Single Array it all works good but when we send multiple objects like more than 2 then we end up in this issue.

When Traced observed that in PZAPPLYDATAFORPERFORMACTION  in Java Step myStepPage.mapInputJson(request, ClipboardPage.PUTALL_MERGE); throwing exception.

Any inputs appreciated.

"errorClassification": "Validation fail",

    "localizedValue": "Validation failed.",

    "errorDetails": [

        {

            "message": "java.lang.IndexOutOfBoundsException: Index 2 out of bounds for length 0",

            "erroneousInputOutputFieldInPage": "",

            "erroneousInputOutputIdentifier": "",

            "errorClassification": "",

            "localizedValue": "java.lang.IndexOutOfBoundsException: Index 2 out of bounds for length 0",

            "messageParameters": []

        }

Branched Information: Originally posted as reply here.