Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I would think it is more a combination of two things:
Mechanical damage (getting pinched by the tweezers)
Lack of oxygen under the substrate.
If it is mechanical damage the part that did not get squished can still grow roots into the substrate, and the plant will likely do fine.
If it is lack of oxygen in the substrate the plant usually does not survive. If you see the brown creeping up the stem take another cutting pretty high up and remove the rotting part. Lay the cutting down on top of, or barely under the substrate, and put a few pebbles on it. Also do something to raise the oxygen level under the substrate.