Dynamic measure calculation using selected date from the slicer

I think the issue might be the ALL, which you don't include in the COUNTROWS version of the measure. Removing that might fix the problem.

I would point out that it is best practice not to filter entire tables but only to filter the columns you need, so you could use

Test Sumx =
VAR SelectedDate = [Selected_Date]
RETURN
    SUMX (
        FILTER (
            SELECTCOLUMNS (
                VW_CONS_PC_AR_LOAD,
                VW_CONS_PC_AR_LOAD[ac_doc_status],
                VW_CONS_PC_AR_LOAD[pstng_date],
                VW_CONS_PC_AR_LOAD[PC_DBCR_GC],
                VW_CONS_PC_AR_LOAD[CLEAR_DATE]
            ),
            ( VW_CONS_PC_AR_LOAD[ac_doc_status] = "O"
                && VW_CONS_PC_AR_LOAD[pstng_date] <= SelectedDate
                && VW_CONS_PC_AR_LOAD[PC_DBCR_GC] <> 0 )
                || ( VW_CONS_PC_AR_LOAD[ac_doc_status] = "C"
                && VW_CONS_PC_AR_LOAD[pstng_date] <= SelectedDate
                && VW_CONS_PC_AR_LOAD[CLEAR_DATE] >= SelectedDate
                && VW_CONS_PC_AR_LOAD[PC_DBCR_GC] <> 0 )
        ),
        1
    )

Finally, your Snowflake query has a GROUP BY clause, which neither of the above measures does, so that will return 1 row per combination of posting date and clearing date, whereas the DAX could return multiple rows.