After editing the field through Textern the edited text does appear in the field, but the webpage behaves as if it was not there.
For example, on Github I can't submit a comment written entirely through Textern until I change something afterwards; if I write something beforehand and then edit through Textern, without further manipulations only stuff that was written before gets submitted.
This also makes Textern unusable on Reddit because it resets the filed to whatever it was before using Textern when I click it.