Hi Jacob,
Amazing catch! It certainly appears to be a bug. I’ll attempt to replicate and log.
As to how missing values are (supposed to be) treated: They are never treated as zero and they don’t appear in plots except in legend coloration where their color is gray. Calculations in plots or attributes ignore missing values. One exception I can think of is the formula: count(!test). Suppose test is an attribute with one missing value. Then the result of this formula will be 1 because the exclamation point signifies negation and returns true for the missing value.