There's no bug at all in HMA. The third color (yellow) was excluded deliberately to avoid a specific linear distortion which occurs when a new value (especially strong) enters or the old one (again strong) leaves the buffer. Under these conditions the yellow part is being relocated back and thus "paints the past" as Nina put it. Sometimes seriously.

And finally it's a matter of preferences. If you like to use a neutral zone as an exit or an early warning then 3 color LSMA is better choice.
Cheers,