After a lot of trial and error, it seems that even if one of the blendtree's animation states isn't shown as active (no blue line leading to it) and even if it should be completely inactive, it can still sometimes have a lingering degree of activity for up to maybe 10 seconds in some cases, which means that any curves or events are still going to be generating values. I found this out while trying to get footsteps to work, and found that the idle animation was still generating events even when nominally inactive. I found a workaround by having the events send a special value indicating that they are from the idle animation clip, but it's an awkward fix.
That sounds like a bug worth reporting. Not having a proper way to determine which clip an event comes from is hella annoying, but I can't think of a better workaround.