Custom Tooltip Not Passing Single-Country Filter Context When Y Axis Uses Conditional Logic

Hi all,

I'm building a line chart in Power BI Desktop (Version 2.152.882.0, March 2026) showing number of students per country by academic year, with a custom tooltip page that should display the student count for the hovered country only. I also have a TopN slider that controls how many countries are visible on the chart.

Setup:
- X Axis: Academic Year
- Y Axis: Headcount (DISTINCTCOUNT of InstanceID)
- Legend: Permanent address[Country]
- Custom tooltip page connected via Format > Visual > Tooltips > Report page
- TopN slider using a GENERATESERIES table

The problem:
The custom tooltip works perfectly when the Y axis uses a plain [Headcount] measure — hovering over a data point passes the correct single-country filter context to the tooltip page.

However, as soon as I use any conditional logic on the Y axis measure (to implement TopN filtering by returning BLANK() for countries outside the TopN), the tooltip stops passing single-country context and instead shows all countries.

I've tried several approaches for the Y axis measure, all of which break the tooltip:

Using TOPN + ALL:

Headcount Filtered =
VAR _n = MAX('TopN'[Value])
VAR _currentCountry = SELECTEDVALUE('Permanent address'[Country])
VAR _topCountries =
TOPN(_n, ALL('Permanent address'[Country]), [Headcount Total (All Years)], DESC)
RETURN
IF(_currentCountry IN _topCountries, [Headcount], BLANK())

Using RANKX:

Headcount Filtered =
VAR _rank =
RANKX(ALL('Permanent address'[Country]), [Headcount Total (All Years)], , DESC, DENSE)
RETURN
IF(_rank <= MAX('TopN'[Value]), [Headcount], BLANK())

Using a calculated column for rank (to avoid ALL in the measure):

-- Calculated column on Permanent address table:
Country Rank =
RANKX(ALL('Permanent address'[Country]), [Headcount Total (All Years)], , DESC, DENSE)

-- Measure:
Headcount Filtered =
IF(MAX('Permanent address'[Country Rank]) <= MAX('TopN'[Value]), [Headcount], BLANK())

Even a completely hardcoded version breaks the tooltip:

Headcount Filtered Test =
IF(MAX('Permanent address'[Country Rank]) <= 10, [Headcount], BLANK())

What I've ruled out:
- The tooltip page has no page-level filters
- The line chart has no visual-level filters (removed Is Top N Country filter from Filters pane)
- The relationship between Permanent address and International Sample data is one-to-many, single direction from Permanent address
- Show items with no data is disabled
- Country is correctly in the Legend well
- The plain [Headcount] measure on the Y axis passes context correctly

The core issue:
It seems like any conditional Y axis measure that returns BLANK() destroys the tooltip filter context, regardless of how simple the condition is. Is this a known limitation? Is there a way to implement TopN country filtering on a line chart without using either the Filters pane or conditional logic on the Y axis measure, so that the custom tooltip context is preserved?

Any help appreciated!