I think this is because of the change in commit 7b00b54 where it's turning the pointer move action into a MouseMove. This is inconsistent with pointer down and pointer up actions for which we get the expected pointerId. I found that while debugging failures of /pointerevents/pointerevent_pointermove_isprimary_same_as_pointerdown.html?touch with a WIP pointer events patch.

cc @yezhizhen