DAX

Hi leejq1 
I have two solutions for it, please try and let me know if it helped you in any way. If MAX is not working for you, you can try the below given methods:

1. DAX

Opening ODO =
VAR CurrentMonthIndex = MAXX(ALLSELECTED('Calendar table'), 'Calendar table'[Index])
VAR LastMonthIndex = CurrentMonthIndex - 1

VAR LatestDateInLastMonth =
    CALCULATE(
        LASTDATE('Shell Report'[Date]),
        FILTER(
            ALL('Calendar table'),
            'Calendar table'[Index] = LastMonthIndex
        ),
        FILTER(
            ALL('Shell Report'),
            'Shell Report'[Vehicle License Number] = MAX('Shell Report'[Vehicle License Number])
        )
    )

VAR OdometerReadingOnLatestDate =
    CALCULATE(
        MAX('Shell Report'[Odometer Reading]),
        'Shell Report'[Date] = LatestDateInLastMonth,
        FILTER(
            ALL('Shell Report'),
            'Shell Report'[Vehicle License Number] = MAX('Shell Report'[Vehicle License Number])
        )
    )

RETURN
    OdometerReadingOnLatestDate

  This should work correctly even with multiple months selected in the filter.

2. DAX

Opening ODO =
VAR CurrentMonthIndex = MAXX(ALLSELECTED('Calendar table'), 'Calendar table'[Index])
VAR LastMonthIndex = CurrentMonthIndex - 1

VAR LatestDateInLastMonth =
    CALCULATETABLE(
        TOPN(
            1,
            FILTER(
                ALL('Shell Report'),
                'Shell Report'[Vehicle License Number] = MAX('Shell Report'[Vehicle License Number])
                && RELATED('Calendar table'[Index]) = LastMonthIndex
            ),
            'Shell Report'[Date],
            DESC
        )
    )

VAR OdometerReadingOnLatestDate =
    CALCULATE(
        MAX('Shell Report'[Odometer Reading]),
        FILTER(
            LatestDateInLastMonth,
            'Shell Report'[Vehicle License Number] = MAX('Shell Report'[Vehicle License Number])
        )
    )

RETURN
    OdometerReadingOnLatestDate

This version with TOPN can be more flexible in situations where MAX or LASTDATE do not provide the intended result across multiple months. Give this a try and let me know how it goes!

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Kind Regards,
Poojara
Data Analyst | MSBI Developer | Power BI Consultant
YouTube: https://youtube.com/@biconcepts?si=04iw9SYI2HN80HKS