My DirectQuery dataset model appears to be using Inner Joins as default

It's me again - the original poster.  I'm wondering whether the issue has something to do with the fact that my Fact table has over a million rows and DirectQuery isn't mad keen on table with over a million rows - could this be affecting the query generated?

If I run Performance analyzer over my analysis then I get this DAX: -

// DAX Query
DEFINE
VAR __DS0FilterTable =
TREATAS({"R45T"}, 'EMEA LISC'[Item_Number])

VAR __DS0FilterTable2 =
TREATAS({"2891"}, 'EMEA LISC'[Business_Unit])

VAR __DS0Core =
SELECTCOLUMNS(
KEEPFILTERS(
FILTER(
KEEPFILTERS(
SUMMARIZECOLUMNS(
'EMEA LISC'[Item_Number],
'EMEA LISC'[Business_Unit],
'EMEA LISC'[JULIANREQUESTDATE_WID],
'Request Date'[Date],
__DS0FilterTable,
__DS0FilterTable2,
"CountRowsEMEA_LISC", CALCULATE(COUNTROWS('EMEA LISC'))
)
),
OR(
OR(
OR(
NOT(ISBLANK('EMEA LISC'[Item_Number])),
NOT(ISBLANK('EMEA LISC'[Business_Unit]))
),
NOT(ISBLANK('EMEA LISC'[JULIANREQUESTDATE_WID]))
),
NOT(ISBLANK('Request Date'[Date]))
)
)
),
"'EMEA LISC'[Item_Number]", 'EMEA LISC'[Item_Number],
"'EMEA LISC'[Business_Unit]", 'EMEA LISC'[Business_Unit],
"'EMEA LISC'[JULIANREQUESTDATE_WID]", 'EMEA LISC'[JULIANREQUESTDATE_WID],
"'Request Date'[Date]", 'Request Date'[Date]
)

VAR __DS0PrimaryWindowed =
TOPN(
501,
__DS0Core,
'EMEA LISC'[Item_Number],
1,
'EMEA LISC'[Business_Unit],
1,
'EMEA LISC'[JULIANREQUESTDATE_WID],
1,
'Request Date'[Date],
1
)

EVALUATE
__DS0PrimaryWindowed

ORDER BY
'EMEA LISC'[Item_Number],
'EMEA LISC'[Business_Unit],
'EMEA LISC'[JULIANREQUESTDATE_WID],
'Request Date'[Date]