Nullable booleans This has been asked before, so just wanted to reiterate that the current behavior in Mixpanel which considers booleans that have null / empty / not set as false is super misleading.
what's the behavior you expect? the idea of what is "falsy" and what is not tends to vary significantly between various languages, there's no standard for it, not even close... coming from python I would expect all three of your examples to be falsy, same in javascript (which is where I'm guessing Mixpanel gets its standard, being a web app and all); however, in ruby, only false and nil evaluate to false...
If I'm sending an event property to indicate that a user "is_registered", then when it's true I'm indicating that I know that she is registered. When it's false I'm indicating that I know that she is not. But if the property is not sent at all with the event, or sent with null, then it means I don't know the registration status. Which is different than knowing that it's false. With other types I can check "is not set", but with boolean I can't, which is very weird and confusing.
as a workaround, you can typecast your boolean to a string, and then you can use the is not set filter
(see also the discussion here https://mixpanelcommunity.slack.com/archives/C032ZAAH6S2/p1743621241693919 about how boolean treatment could be better in general)