This is a side effect of how user properties are updated in Amplitude: by default you only need to set them once or when they change, so when the value of the property is null, Amplitude assumes it has not changed and keeps the previous value.

I would suggest using some non-null value in the user property for these users. “Undefined” or something like this. You can still filter these values out of any charts you make, and it also allows you to differentiate between users that had the property set at some point and those that have never had it set in the first place.

If a user property is set to an empty object (after having been set to some normal value, say, `true`), does it behave the same way as if trying to update with `null`? Or will Amplitude update the value to an empty object?