A strange issue happened when using If(Aggr(Only()))

[Alex_LYQ]

I'm using below dimension and measure to build a chart

Dimension:

=IF(GetSelectedCount(ObjectUsageType)=1,
Pick(Match(GetFieldSelections(ObjectUsageType), 'Object', 'Field', 'Export'),
ObjectName,
[Selected Field],
ObjectName
),
Aggr(Only({<ObjectUsageType={"Object"}>}ObjectName), ObjectName)
)

Measure:

NUM(SUM({<Flag_Session={$(vShowUsage)}>}ObjectUsageCount))

related data model:

In this case, I found that when I'm using this dimension and measure, the value of measure looks not correct, and when I click one of the dimension, it will auto select a random value of field [Selected Field]:

But I didn't filter any value of [Selected Field], it should display all the value of [Selected Field] right?

And then I found that if I changed the Dimension to below:

=Aggr(Only({<ObjectUsageType={"Object"}>}ObjectName), ObjectName)

then it can display correctly

The only difference is whether I add the if condition, but this condition normally takes the second expression. Why does it cause such a difference?