Unusual Filter Behavior - Filter Context Not Being Removed as Expected

I have a Sales fact table with the following fields:

[Sales Amount]

[Sales Quantity]

[Item]

[Item Revision]

[Sales Period] (a text field in the format of YYMM)

[Customer]

[Sales Region]

I have a measure:

[Sales Total] := SUM(Sales[Sales Amount]) 

and another measure

[Sales Total All Items] := CALCULATE(
    SUM(Sales[Sales Amount]),
    ALL(Sales[Item], Sales[Revision])
)

and finally

[Item Revision % of Total Sales] := DIVIDE(
    [Sales Total],
    [Sales Total All Items])

I have a page-level filter on [Sales Region], and I have a slicer on [Customer] and [Period]. I also have a stacked bar chart visual with [Sales Amount] by [Item]. Finally, I have two tables: one with data aggregated by [Period], and one with transaction-level data.

The problem is that when I have more than one [Customer]s selected in my slicer, and I filter by [Item] using my visual, my [Sales Total All Items] measure will sum the rows only where [Customer] purchased the filtered [Item].

For example, if I had two [Customers], "A" that purchased the selected [Item], and "B" that didn't, [Sales Total All Items] will only reflect the total of all items from "A." Likewise, if I filter on another [Item] that both had purchased, [Sales Total All Items] will reflect the same amount as if I had not applied any filters on [Item].

If I "Select All" in my [Customer] slicer, I can filter on [Item] and get an appropriate total across all [Customers]. Likewise, when I de-select my [Item] filter in my visual, I once again get a correct total in my aggregate table.

When I filter on [Sales Period] and apply the same [Item] filter, I see the same behavior: [Sales Total All Items] only provides a total for the [Sales Period]s in which there's a record for the filtered [Item].

To summarize, even though my measure is using CALCULATE( . . ., ALL([Item]) ) to remove filter context, it still seems to be interacting with my other filters. I don't really understand this behavior, and I don't know how to get around it.

I appreciate any help.