Not sure this has been reported yet, so here it is.
See attached pic, taken after doing
Edit->Filter->Voices->All stem-up Voices
Stem-down notes are still selected (although the rests are un-selected.)
Filters for voices on percussion kits are a bit wonky, because of the way drum kits are constructed from multiple voices taken from individual instruments consolidated together onto a single staff. We hope to provide some further improvements in this area in the future.
That’s what I thought. Thanks!