Drill Down the Row Hierarchy in Custom Matrix Visual

Yes, if you enable drilldown capabilities, then Power BI is only going to send the data that the user has requested to view, which may not be all of it, or it could be a specific level only (which could also be an interim level of a multilevel hierarchy if you're not using a matrix data view mapping and the user is drilling rather than expanding).

I can't speak for combining drill capabilities with filtering/slicing, as I personally avoid doing work with slicers; I have been burned too many times by their limitations, and what this allows me to deliver for clients without significant workarounds, realistically. I would assume that, as the selection/drill APIs and the filter APIs have their own invocation methods, they theoretically could coexist.

The reality of whether this could happen *feels* like a significant problem to solve, and I think it would require a lot more time and effort than I can dedicate to a forum post, or my current capacity would allow. Off the top of my head, I might start by ensuring that something in my formula memoizes or will enable me to discover what level or node the formula applies to, and when the data view is (re) processed, this will need to be taken into consideration to ensure that the correct expansion state is applied programmatically if not already there.

Assuming you are using the matrix data view mapping, you could do this part by selection ID and programmatically expanding the nodes you need with ISelectionManager.toggleExpandCollapse(). With a categorical mapping, this might be achievable with the drilldown API, but it's less targeted than you can do for the matrix mapping.

Hopefully some of this may help, but you may get a definitive answer if you speak directly with the support team and provide them with your scenario.

Good luck,

Daniel