SUMX over table variable breaks when adding relationships to underlying table - powerbi

Question

Environment
I have created the two queries in Power Query
"Demo"
let
Source = Table.FromList(List.Random(1000, 20200427),Splitter.SplitByNothing()),
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),
#"Split Column by Position" = Table.SplitColumn(#"Changed Type", "Column1", Splitter.SplitTextByPositions({0, 5, 7}, false), {"Month", "Amount"}),
#"Changed Type2" = Table.TransformColumnTypes(#"Split Column by Position",{{"Month", type number}, {"Amount", type number}}),
ConvetToMonthNumber = Table.TransformColumns(#"Changed Type2",{{"Month", each Number.RoundDown( 12 * _) + 1, Int64.Type}})
in
ConvetToMonthNumber
and "MockCal"
let
Source = Table.FromList( List.Repeat({1..4},3), Splitter.SplitByNothing(),{"CalQuart"}),
#"Added Index" = Table.AddIndexColumn(Source, "Month", 1, 1)
in
#"Added Index"
and then in the model I have two bits of DAX
Running Amount =
CALCULATE(
SUM('Demo'[Amount]),
FILTER(ALL('Demo'[Month]), 'Demo'[Month] <= MAX('Demo'[Month]))
)
and
WEIRD Run total =
VAR CalcTable = SUMMARIZE(Demo,Demo[Month],"MonthlyRollingAmounts",[Running Amount])
VAR TotalAmount = SUMX(CalcTable,[MonthlyRollingAmounts])
RETURN IF(ISFILTERED(Demo[Month]), [Running Amount], TotalAmount)
With this I can produce the following visual:
This achieves the desired result: Create a measure that sums up its slices by month.
However, when I create a relationship off this table, the SUMX stops working as expected.
The question is: Why does adding this relationship change the behavior of the SUMX? It doesn't seem to me like it should matter at all. Help is appreciated, this one is really bending my brain.

Answer 1

The reason for this is that in your Running Amount measure you remove any filtering on 'Demo'[Month] but this does not propagate upstream to MockCal (which I'm assuming you're using as the first column in your visual).
In general, you want to do your date filtering on your calendar table instead of your fact table. Try this instead:
Running Amount =
CALCULATE(
SUM('Demo'[Amount]),
FILTER(ALL(MockCal), 'MockCal'[Month] <= MAX('MockCal'[Month]))
)
Your WEIRD Run total should be updated as well to use the calendar table.