Uneven lowercase at low resolution

Here is the screenshot of the typeface I am working on. At 19px size and lower, lowercase start to "dance". Tops and bottoms are uneven, like they have a different baseline/x-height. I don't know what's the reason, because I can't recall such situations earlier. I defined the zones in a classic manner, and did PS autohinting (with manual hint adjustment in addition). Even tried to push extreme nodes in zones (thought it might be a coordinate rounding problem). I would appreciate much any input on this, thanks!