Selectivity of etch method is lost when depth (overetch) is too large

[resuSover]

I believe there is a bug with selective etch method when etch depth is too large and etch is done through sandwich of layers. It looses its selectivity and layers in between do not act as protectors from air anymore.

Example:

#defining all dark_blue_layers
only_dark_blue_layers = [@top_dark_blue, @bottom_dark_blue]

#Etching into darkblue layers only, so etch should stop if encounters other layers
_overetch = 0.01 #if overetch is too large (e.g. 0.1), then it will also etch the bottom darkblue layer which it should not, since that one is protected by green_middle_layer
_depth = _top_dark_blue_thickness + _overetch
etch(_depth, 0, :mode => :round, :into => only_dark_blue_layers)
If in this example I make overetch too large, then etch method starts to ignore green_middle_layer and will etch the bottom darkblue despite that it has no access to it from "air" since it is protected by green_middle_layer.

Starting layer configuration:

Small overetch (smaller than the thickness of green_middle_layer) results in correctly selective etch:

Large overetch (larger than the thickness of green_middle_layer) results in losing selectivity and not respecting green_layer air-protection anymore and this leads to hanging layer like this: