Layer when it isn't top layer doesn't stop etch

[mwdvorak]

Hi,
It appears that when there's a layer above a non-etching layer, then the etch goes through as if the non-etching layer wasn't there.

depth(1)
height(6)

dbu(0.0001)
delta(10 * dbu)

lmetal1 = layer("18/0")
lvia = layer("21/0")
substrate = bulk

metal1 = mask(lmetal1).grow(0.2)
oxide = all.grow(1)
mask(lvia).etch(1.4, :into => [substrate, oxide] )

output("substrate/0", substrate)
output("oxide/0",oxide)
output("metal1/0", metal1)

If you remove the top oxide layer, the etch won't go through metal1, but if it's present, it will.

Is this a known feature of Xsection?